diff --git a/.travis.yml b/.travis.yml
index 795c640..ed499e8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,7 +14,6 @@ env:
     - TEST_FILE=empy
     - TEST_FILE=fava
     - TEST_FILE=flake8
-    - TEST_FILE=flake8_mercurial
     - TEST_FILE=flit
     - TEST_FILE=ldap
     - TEST_FILE=lektor
@@ -29,7 +28,7 @@ env:
     - TEST_FILE=tornado
 install:
 - sudo mount -o remount,exec,size=4G,mode=755 /run/user || true
-- nix-env -iA nixpkgs.nix-prefetch-git nixpkgs.nix-prefetch-hg
+- nix-env -iA nixpkgs.nix-prefetch-git
 before_script:
 - sudo mkdir -p /etc/nix && echo 'sandbox = true' | sudo tee /etc/nix/nix.conf
 script:
diff --git a/integrationtests/flake8-mercurial/requirements_override.nix b/integrationtests/flake8-mercurial/requirements_override.nix
deleted file mode 100644
index fb40243..0000000
--- a/integrationtests/flake8-mercurial/requirements_override.nix
+++ /dev/null
@@ -1,9 +0,0 @@
-{ pkgs, python }:
-
-self: super: {
-  "pytest-runner" = super."pytest-runner".overrideDerivation
-    (old: { buildInputs = old.buildInputs ++ [ self."setuptools-scm" ]; });
-
-  "mccabe" = super.mccabe.overrideDerivation
-    (old: { buildInputs = old.buildInputs ++ [ self."pytest-runner" ]; });
-}
diff --git a/integrationtests/test_aiohttp.py b/integrationtests/test_aiohttp.py
index ff73611..a8dba89 100644
--- a/integrationtests/test_aiohttp.py
+++ b/integrationtests/test_aiohttp.py
@@ -5,4 +5,4 @@ class AiohttpTestCase(IntegrationTest):
     name_of_testcase = "aiohttp"
     code_for_testing = ["import aiohttp"]
     requirements = ["aiohttp==2.0.6.post1"]
-    python_version = "python35"
+    python_version = "python37"
diff --git a/integrationtests/test_flake8_mercurial.py b/integrationtests/test_flake8_mercurial.py
deleted file mode 100644
index 45df13b..0000000
--- a/integrationtests/test_flake8_mercurial.py
+++ /dev/null
@@ -1,19 +0,0 @@
-from .framework import IntegrationTest
-
-REVISION = "a209fb6"
-
-
-class Flake8MercurialTestCase(IntegrationTest):
-    name_of_testcase = "flake8-mercurial"
-    code_for_testing = ["import flake8"]
-    requirements = [
-        "-e hg+https://bitbucket.org/tarek/flake8@{revision}#egg=flake8".format(
-            revision=REVISION
-        )
-    ]
-
-    def setup_requires(self):
-        return ["setuptools-scm", "pytest-runner"]
-
-    def requirements_file_check(self, content):
-        self.assertIn(REVISION, content)
diff --git a/requirements.nix b/requirements.nix
index 3b4c0b7..de5fa77 100644
--- a/requirements.nix
+++ b/requirements.nix
@@ -12,14 +12,9 @@
 let
 
   inherit (pkgs) makeWrapper;
-  inherit (pkgs.stdenv.lib) fix' extends inNixShell;
+  inherit (pkgs.lib) fix' extends inNixShell;
 
-  pythonPackages =
-  import "${toString pkgs.path}/pkgs/top-level/python-packages.nix" {
-    inherit pkgs;
-    inherit (pkgs) stdenv;
-    python = pkgs.python3;
-  };
+  pythonPackages = pkgs.python3.pkgs;
 
   commonBuildInputs = with pkgs; [ openssl libffi ];
   commonDoCheck = false;
@@ -86,7 +81,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://alabaster.readthedocs.io";
         license = licenses.bsdOriginal;
         description = "A configurable sidebar-enabled Sphinx theme";
@@ -103,7 +98,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://github.com/ActiveState/appdirs";
         license = licenses.mit;
         description = "A small Python module for determining appropriate platform-specific dirs, e.g. a "user data dir".";
@@ -123,7 +118,7 @@ let
         self."wheel"
       ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://www.attrs.org/";
         license = licenses.mit;
         description = "Classes Without Boilerplate";
@@ -142,7 +137,7 @@ let
       propagatedBuildInputs = [
         self."pytz"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://babel.pocoo.org/";
         license = licenses.bsdOriginal;
         description = "Internationalization utilities";
@@ -171,7 +166,7 @@ let
         self."toml"
         self."typed-ast"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/psf/black";
         license = licenses.mit;
         description = "The uncompromising code formatter.";
@@ -191,7 +186,7 @@ let
         self."six"
         self."webencodings"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/mozilla/bleach";
         license = licenses.asl20;
         description = "An easy safelist-based HTML-sanitizing tool.";
@@ -211,7 +206,7 @@ let
         self."click"
         self."pyaml"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/kylie-a/bumpv";
         license = licenses.mit;
         description = "Version-bump your software with a single command!";
@@ -228,7 +223,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://certifi.io/";
         license = licenses.mpl20;
         description = "Python package for providing Mozilla's CA Bundle.";
@@ -247,7 +242,7 @@ let
       propagatedBuildInputs = [
         self."pycparser"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://cffi.readthedocs.org";
         license = licenses.mit;
         description = "Foreign Function Interface for Python calling C code.";
@@ -264,7 +259,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/chardet/chardet";
         license = licenses.lgpl2;
         description = "Universal encoding detector for Python 2 and 3";
@@ -281,7 +276,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://palletsprojects.com/p/click/";
         license = licenses.bsdOriginal;
         description = "Composable command line interface toolkit";
@@ -298,7 +293,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/nedbat/coveragepy";
         license = licenses.asl20;
         description = "Code coverage measurement for Python";
@@ -322,7 +317,7 @@ let
         self."cffi"
         self."six"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pyca/cryptography";
         license = licenses.asl20;
         description = "cryptography is a package which provides cryptographic recipes and primitives to Python developers.";
@@ -339,7 +334,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://docutils.sourceforge.net/";
         license = licenses.publicDomain;
         description = "Docutils -- Python Documentation Utilities";
@@ -358,7 +353,7 @@ let
       propagatedBuildInputs = [
         self."attrs"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://github.com/python-effect/effect/";
         license = licenses.mit;
         description = "pure effects for Python";
@@ -377,7 +372,7 @@ let
         self."flit"
       ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/takluyver/entrypoints";
         license = licenses.mit;
         description = "Discover and load entry points from installed packages.";
@@ -396,7 +391,7 @@ let
       propagatedBuildInputs = [
         self."pyrepl"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pdbpp/fancycompleter";
         license = licenses.bsdOriginal;
         description = "colorful TAB completion for Python prompt";
@@ -418,7 +413,7 @@ let
         self."pycodestyle"
         self."pyflakes"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://gitlab.com/pycqa/flake8";
         license = licenses.mit;
         description = "the modular source code checker: pep8, pyflakes and co";
@@ -438,7 +433,7 @@ let
         self."flake8"
         self."pycodestyle"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/jbkahn/flake8-debugger";
         license = licenses.mit;
         description = "ipdb/pdb statement checker plugin for flake8";
@@ -457,7 +452,7 @@ let
       propagatedBuildInputs = [
         self."flake8"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/nhoad/flake8-unused-arguments";
         license = licenses.mit;
         description = "flake8 extension to warn on unused function arguments";
@@ -481,7 +476,7 @@ let
         self."pytoml"
         self."requests"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/takluyver/flit";
         license = licenses.bsdOriginal;
         description = "A simple packaging tool for simple packages.";
@@ -502,7 +497,7 @@ let
       propagatedBuildInputs = [
         self."pytoml"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/takluyver/flit";
         license = licenses.bsdOriginal;
         description = "Distribution-building parts of Flit. See flit package for more information";
@@ -522,7 +517,7 @@ let
         self."attrs"
         self."sortedcontainers"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/HypothesisWorks/hypothesis/tree/master/hypothesis-python";
         license = licenses.mpl20;
         description = "A library for property-based testing";
@@ -539,7 +534,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/kjd/idna";
         license = licenses.bsdOriginal;
         description = "Internationalized Domain Names in Applications (IDNA)";
@@ -556,7 +551,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/shibukawa/imagesize_py";
         license = licenses.mit;
         description = "Getting image size from png/jpeg/jpeg2000/gif file";
@@ -579,7 +574,7 @@ let
       propagatedBuildInputs = [
         self."zipp"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://importlib-metadata.readthedocs.io/";
         license = licenses.asl20;
         description = "Read metadata from Python packages";
@@ -598,7 +593,7 @@ let
       propagatedBuildInputs = [
         self."pytoml"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/takluyver/intreehooks";
         license = licenses.mit;
         description = "Load a PEP 517 backend from inside the source tree";
@@ -615,7 +610,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/timothycrosley/isort";
         license = licenses.mit;
         description = "A Python utility / library to sort Python imports.";
@@ -634,7 +629,7 @@ let
         self."flit-core"
       ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://gitlab.com/takluyver/jeepney";
         license = licenses.mit;
         description = "Low-level, pure Python DBus protocol wrapper.";
@@ -653,7 +648,7 @@ let
       propagatedBuildInputs = [
         self."markupsafe"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://palletsprojects.com/p/jinja/";
         license = licenses.bsdOriginal;
         description = "A very fast and expressive template engine.";
@@ -680,7 +675,7 @@ let
         self."setuptools"
         self."six"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/Julian/jsonschema";
         license = licenses.mit;
         description = "An implementation of JSON Schema validation for Python";
@@ -705,7 +700,7 @@ let
         self."jeepney"
         self."secretstorage"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/jaraco/keyring";
         license = licenses.mit;
         description = "Store and access your passwords safely.";
@@ -722,7 +717,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://palletsprojects.com/p/markupsafe/";
         license = licenses.bsdOriginal;
         description = "Safely add untrusted strings to HTML/XML markup.";
@@ -739,7 +734,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pycqa/mccabe";
         license = licenses.mit;
         description = "McCabe checker, plugin for flake8";
@@ -756,7 +751,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/erikrose/more-itertools";
         license = licenses.mit;
         description = "More routines for operating on iterables, beyond itertools";
@@ -777,7 +772,7 @@ let
         self."typed-ast"
         self."typing-extensions"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://www.mypy-lang.org/";
         license = licenses.mit;
         description = "Optional static typing for Python";
@@ -794,7 +789,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/python/mypy_extensions";
         license = licenses.mit;
         description = "Experimental type system extensions for programs checked with the mypy typechecker.";
@@ -816,7 +811,7 @@ let
         self."effect"
         self."jinja2"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/seppeljordan/nix-prefetch-github";
         license = "GPLv3";
         description = "Prefetch source code from github for nix build tool";
@@ -836,7 +831,7 @@ let
         self."pyparsing"
         self."six"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pypa/packaging";
         license = licenses.asl20;
         description = "Core utilities for Python packages";
@@ -853,7 +848,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://launchpad.net/parsley";
         license = licenses.mit;
         description = "Parsing and pattern matching made easy.";
@@ -870,7 +865,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/cpburnz/python-path-specification";
         license = licenses.mpl20;
         description = "Utility library for gitignore style pattern matching of file paths.";
@@ -891,7 +886,7 @@ let
         self."pygments"
         self."wmctrl"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://github.com/antocuni/pdb";
         license = licenses.bsdOriginal;
         description = "pdb++, a drop-in replacement for pdb";
@@ -908,7 +903,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://code.launchpad.net/~tseaver/pkginfo/trunk";
         license = licenses.mit;
         description = "Query metadatdata from sdists / bdists / installed packages.";
@@ -931,7 +926,7 @@ let
       propagatedBuildInputs = [
         self."importlib-metadata"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pytest-dev/pluggy";
         license = licenses.mit;
         description = "plugin and hook calling mechanisms for python";
@@ -948,7 +943,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://py.readthedocs.io/";
         license = licenses.mit;
         description = "library with cross-python path, ini-parsing, io, code, log facilities";
@@ -967,7 +962,7 @@ let
       propagatedBuildInputs = [
         self."pyyaml"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/mk-fg/pretty-yaml";
         license = "WTFPL";
         description = "PyYAML-based module to produce pretty and readable YAML-serialized data";
@@ -984,7 +979,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://pycodestyle.readthedocs.io/";
         license = licenses.mit;
         description = "Python style guide checker";
@@ -1001,7 +996,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/eliben/pycparser";
         license = licenses.bsdOriginal;
         description = "C parser in Python";
@@ -1018,7 +1013,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/PyCQA/pyflakes";
         license = licenses.mit;
         description = "passive checker of Python programs";
@@ -1035,7 +1030,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://pygments.org/";
         license = licenses.bsdOriginal;
         description = "Pygments is a syntax highlighting package written in Python.";
@@ -1052,7 +1047,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pyparsing/pyparsing/";
         license = licenses.mit;
         description = "Python parsing module";
@@ -1069,7 +1064,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://bitbucket.org/pypy/pyrepl/";
         license = "MIT X11 style";
         description = "A library for building flexible command line interfaces";
@@ -1088,7 +1083,7 @@ let
       propagatedBuildInputs = [
         self."six"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://github.com/tobgu/pyrsistent/";
         license = licenses.mit;
         description = "Persistent/Functional/Immutable data structures";
@@ -1117,7 +1112,7 @@ let
         self."py"
         self."wcwidth"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://docs.pytest.org/en/latest/";
         license = licenses.mit;
         description = "pytest: simple powerful testing with Python";
@@ -1137,7 +1132,7 @@ let
         self."coverage"
         self."pytest"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pytest-dev/pytest-cov";
         license = licenses.bsdOriginal;
         description = "Pytest plugin for measuring coverage.";
@@ -1158,7 +1153,7 @@ let
         self."wheel"
       ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pytest-dev/pytest-runner/";
         license = licenses.mit;
         description = "Invoke py.test as distutils command with dependency resolution";
@@ -1175,7 +1170,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/avakar/pytoml";
         license = licenses.mit;
         description = "A parser for TOML-0.4.0";
@@ -1192,7 +1187,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://pythonhosted.org/pytz";
         license = licenses.mit;
         description = "World timezone definitions, modern and historical";
@@ -1209,7 +1204,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/yaml/pyyaml";
         license = licenses.mit;
         description = "YAML parser and emitter for Python";
@@ -1231,7 +1226,7 @@ let
         self."pygments"
         self."six"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pypa/readme_renderer";
         license = licenses.asl20;
         description = "readme_renderer is a library for rendering "readme" descriptions for Warehouse";
@@ -1248,7 +1243,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://bitbucket.org/mrabarnett/mrab-regex";
         license = licenses.psfl;
         description = "Alternative regular expression module, to replace re.";
@@ -1270,7 +1265,7 @@ let
         self."idna"
         self."urllib3"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://requests.readthedocs.io";
         license = licenses.asl20;
         description = "Python HTTP for Humans.";
@@ -1289,7 +1284,7 @@ let
       propagatedBuildInputs = [
         self."requests"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://toolbelt.readthedocs.org";
         license = licenses.asl20;
         description = "A utility belt for advanced users of python-requests";
@@ -1312,7 +1307,7 @@ let
         self."cryptography"
         self."jeepney"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/mitya57/secretstorage";
         license = licenses.bsdOriginal;
         description = "Python bindings to FreeDesktop.org Secret Service API";
@@ -1329,7 +1324,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/zsimic/setupmeta";
         license = licenses.mit;
         description = "Simplify your setup.py";
@@ -1346,7 +1341,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pypa/setuptools";
         license = licenses.mit;
         description = "Easily download, build, install, upgrade, and uninstall Python packages";
@@ -1366,7 +1361,7 @@ let
         self."wheel"
       ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pypa/setuptools_scm/";
         license = licenses.mit;
         description = "the blessed package to manage your versions by scm tags";
@@ -1383,7 +1378,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/benjaminp/six";
         license = licenses.mit;
         description = "Python 2 and 3 compatibility utilities";
@@ -1400,7 +1395,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/snowballstem/snowball";
         license = licenses.bsdOriginal;
         description = "This package provides 26 stemmers for 25 languages generated from Snowball algorithms.";
@@ -1417,7 +1412,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://www.grantjenks.com/docs/sortedcontainers/";
         license = licenses.asl20;
         description = "Sorted Containers -- Sorted List, Sorted Dict, Sorted Set";
@@ -1451,7 +1446,7 @@ let
         self."sphinxcontrib-qthelp"
         self."sphinxcontrib-serializinghtml"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://sphinx-doc.org/";
         license = licenses.bsdOriginal;
         description = "Python documentation generator";
@@ -1468,7 +1463,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://sphinx-doc.org/";
         license = licenses.bsdOriginal;
         description = "sphinxcontrib-applehelp is a sphinx extension which outputs Apple help books";
@@ -1485,7 +1480,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://sphinx-doc.org/";
         license = licenses.bsdOriginal;
         description = "sphinxcontrib-devhelp is a sphinx extension which outputs Devhelp document.";
@@ -1502,7 +1497,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://sphinx-doc.org/";
         license = licenses.bsdOriginal;
         description = "sphinxcontrib-htmlhelp is a sphinx extension which renders HTML help files";
@@ -1519,7 +1514,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://sphinx-doc.org/";
         license = licenses.bsdOriginal;
         description = "A sphinx extension which renders display math in HTML via JavaScript";
@@ -1536,7 +1531,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://sphinx-doc.org/";
         license = licenses.bsdOriginal;
         description = "sphinxcontrib-qthelp is a sphinx extension which outputs QtHelp document.";
@@ -1553,7 +1548,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://sphinx-doc.org/";
         license = licenses.bsdOriginal;
         description = "sphinxcontrib-serializinghtml is a sphinx extension which outputs "serialized" HTML files (json and pickle).";
@@ -1570,7 +1565,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/uiri/toml";
         license = licenses.mit;
         description = "Python Library for Tom's Obvious, Minimal Language";
@@ -1587,7 +1582,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/tqdm/tqdm";
         license = licenses.mit;
         description = "Fast, Extensible Progress Meter";
@@ -1617,7 +1612,7 @@ let
         self."setuptools"
         self."tqdm"
       ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://twine.readthedocs.io/";
         license = licenses.asl20;
         description = "Collection of utilities for publishing packages on PyPI";
@@ -1634,7 +1629,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/python/typed_ast";
         license = licenses.asl20;
         description = "a fork of Python 2 and 3 ast modules with type comment support";
@@ -1651,7 +1646,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/python/typing/blob/master/typing_extensions/README.rst";
         license = licenses.psfl;
         description = "Backported and Experimental Type Hints for Python 3.5+";
@@ -1668,7 +1663,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://urllib3.readthedocs.io/";
         license = licenses.mit;
         description = "HTTP library with thread-safe connection pooling, file post, and more.";
@@ -1685,7 +1680,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/jquast/wcwidth";
         license = licenses.mit;
         description = "Measures number of Terminal column cells of wide-character codes";
@@ -1702,7 +1697,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/SimonSapin/python-webencodings";
         license = licenses.bsdOriginal;
         description = "Character encoding aliases for legacy web content";
@@ -1721,7 +1716,7 @@ let
         self."setuptools"
       ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/pypa/wheel";
         license = licenses.mit;
         description = "A built-package format for Python";
@@ -1738,7 +1733,7 @@ let
       format = "setuptools";
       buildInputs = commonBuildInputs ++ [ ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "http://bitbucket.org/antocuni/wmctrl";
         license = licenses.bsdOriginal;
         description = "A tool to programmatically control windows inside X";
@@ -1759,7 +1754,7 @@ let
         self."wheel"
       ];
       propagatedBuildInputs = [ ];
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "https://github.com/jaraco/zipp";
         license = licenses.mit;
         description = "Backport of pathlib-compatible object wrapper for zip files";
@@ -1784,4 +1779,4 @@ in python.withPackages
             generated
             allOverrides
          )
-   )
\ No newline at end of file
+   )
diff --git a/shell.nix b/shell.nix
index 741aba4..8a14656 100644
--- a/shell.nix
+++ b/shell.nix
@@ -8,6 +8,7 @@ nixpkgs.mkShell {
     nixfmt
     git
     nix-prefetch-hg
+    nix-prefetch-git
   ];
   shellHook = ''
     export PATH=${./scripts}:$PATH
diff --git a/src/pypi2nix/pip/virtualenv.py b/src/pypi2nix/pip/virtualenv.py
index cdd34dc..f131183 100644
--- a/src/pypi2nix/pip/virtualenv.py
+++ b/src/pypi2nix/pip/virtualenv.py
@@ -41,6 +41,9 @@ def __init__(
 
     def prepare_virtualenv(self) -> None:
         self.env_builder.create(self.target_directory)
+        self._execute_venv_python_command(
+            ["-m", "ensurepip"]
+        )
         self._execute_pip_command(
             ["install", self._wheel_requirement_name()] + self._maybe_index()
         )
@@ -106,7 +109,7 @@ def freeze(self, python_path: List[Path]) -> str:
         return self._execute_pip_command(["freeze"], pythonpath=python_path)
 
     def _pip_path(self) -> str:
-        return os.path.join(self.target_directory, "bin", "pip")
+        return os.path.join(self.target_directory, "bin", "pip3")
 
     def _execute_pip_command(
         self, arguments: List[str], pythonpath: List[Path] = []
@@ -119,6 +122,19 @@ def _execute_pip_command(
             raise PipFailed(output=output)
         return output
 
+    def _execute_venv_python_command(
+        self, arguments: List[str], pythonpath: List[Path] = []
+    ) -> str:
+        with self._explicit_pythonpath(pythonpath), self._set_environment_variable(
+            {"SOURCE_DATE_EPOCH": "315532800",}
+        ):
+            returncode, output = cmd(
+                [os.path.join(self.target_directory, "bin", "python")] + arguments, logger=self.logger
+            )
+        if returncode != 0:
+            raise PipFailed(output=output)
+        return output
+
     @contextmanager
     def _explicit_pythonpath(self, pythonpath: List[Path]) -> Iterator[None]:
         additional_paths = ":".join(map(str, pythonpath))
diff --git a/src/pypi2nix/python_version.py b/src/pypi2nix/python_version.py
index b9d082a..8336f7f 100644
--- a/src/pypi2nix/python_version.py
+++ b/src/pypi2nix/python_version.py
@@ -7,7 +7,6 @@
 
 @unique
 class PythonVersion(Enum):
-    python35 = "python35"
     python36 = "python36"
     python37 = "python37"
     python38 = "python38"
@@ -24,7 +23,6 @@ def major_version(self) -> str:
 
 
 _PYTHON_VERSIONS: Dict[str, PythonVersion] = {
-    "3.5": PythonVersion.python35,
     "3.6": PythonVersion.python36,
     "3.7": PythonVersion.python37,
     "3.8": PythonVersion.python38,
diff --git a/src/pypi2nix/requirements.py b/src/pypi2nix/requirements.py
index 87573f5..9348f05 100644
--- a/src/pypi2nix/requirements.py
+++ b/src/pypi2nix/requirements.py
@@ -115,7 +115,7 @@ def add(self, other: Requirement, target_platform: TargetPlatform) -> Requiremen
                     )
                 )
             elif isinstance(other, UrlRequirement):
-                if self.url != other.url:
+                if self._url != other._url:
                     raise IncompatibleRequirements(
                         "Cannot combine requirements with different urls `{url1}` and `{url2}`".format(
                             url1=self.url, url2=other.url
@@ -215,10 +215,10 @@ def add(self, other: Requirement, target_platform: TargetPlatform) -> Requiremen
                     )
                 )
             elif isinstance(other, PathRequirement):
-                if self.path != other.path:
+                if self._path != other._path:
                     raise IncompatibleRequirements(
                         "Cannot combine requirements with different paths `{path1}` and `{path2}`".format(
-                            path1=self.path, path2=other.path
+                            path1=self._path, path2=other._path
                         )
                     )
                 else:
diff --git a/src/pypi2nix/templates/generated.nix.j2 b/src/pypi2nix/templates/generated.nix.j2
index fe18ecb..71cd58c 100644
--- a/src/pypi2nix/templates/generated.nix.j2
+++ b/src/pypi2nix/templates/generated.nix.j2
@@ -5,7 +5,7 @@
       format = "{{ package_format }}";
       buildInputs = commonBuildInputs ++ {{ buildInputs }};
       propagatedBuildInputs = {{ propagatedBuildInputs }};
-      meta = with pkgs.stdenv.lib; {
+      meta = with pkgs.lib; {
         homepage = "{{ homepage }}";
         license = {{ license }};
         description = "{{ description }}";
diff --git a/src/pypi2nix/templates/requirements.nix.j2 b/src/pypi2nix/templates/requirements.nix.j2
index b5180f2..16172ee 100644
--- a/src/pypi2nix/templates/requirements.nix.j2
+++ b/src/pypi2nix/templates/requirements.nix.j2
@@ -12,14 +12,9 @@
 let
 
   inherit (pkgs) makeWrapper;
-  inherit (pkgs.stdenv.lib) fix' extends inNixShell;
+  inherit (pkgs.lib) fix' extends inNixShell;
 
-  pythonPackages =
-  import "${toString pkgs.path}/pkgs/top-level/python-packages.nix" {
-    inherit pkgs;
-    inherit (pkgs) stdenv;
-    python = pkgs.{{ python_version }};
-  };
+  pythonPackages = pkgs.{{ python_version }}.pkgs;
 
   commonBuildInputs = {{ extra_build_inputs }};
   commonDoCheck = {{ enable_tests }};
diff --git a/unittests/test_package_generator.py b/unittests/test_package_generator.py
index dc007c9..8e03935 100644
--- a/unittests/test_package_generator.py
+++ b/unittests/test_package_generator.py
@@ -104,7 +104,7 @@ def pip(
         logger=logger,
         target_platform=current_platform,
         target_directory=str(install_target),
-        env_builder=venv.EnvBuilder(with_pip=True),
+        env_builder=venv.EnvBuilder(with_pip=True, symlinks=True),
         no_index=True,
         wheel_distribution_path=wheel_distribution_archive_path,
         find_links=[str(target_directory)],
diff --git a/unittests/test_package_source.py b/unittests/test_package_source.py
index 7c4a0fb..0c625eb 100644
--- a/unittests/test_package_source.py
+++ b/unittests/test_package_source.py
@@ -2,7 +2,6 @@
 
 from pypi2nix.nix import Nix
 from pypi2nix.package_source import GitSource
-from pypi2nix.package_source import HgSource
 from pypi2nix.package_source import PathSource
 from pypi2nix.package_source import UrlSource
 
@@ -20,13 +19,6 @@ def git_source():
     )
 
 
-@pytest.fixture
-def hg_source(logger):
-    return HgSource(
-        url="https://bitbucket.org/tarek/flake8", revision="a209fb69350c", logger=logger
-    )
-
-
 @pytest.fixture
 def url_source(logger):
     return UrlSource(url=URL_SOURCE_URL, logger=logger)
@@ -58,18 +50,6 @@ def test_git_source_produces_valid_nix_expression(git_source, expression_evaluat
     expression_evaluater(git_source.nix_expression())
 
 
-@nix
-def test_hg_source_gives_correct_hash_value(hg_source):
-    assert (
-        hg_source.hash_value() == "1n0fzlzmfmynnay0n757yh3qwjd9xxcfi7vq4sxqvsv90c441s7v"
-    )
-
-
-@nix
-def test_hg_source_produces_valid_nix_expression(hg_source, expression_evaluater):
-    expression_evaluater(hg_source.nix_expression())
-
-
 @nix
 def test_url_source_gives_correct_hash_value(url_source):
     assert url_source.hash_value() == URL_SOURCE_HASH
diff --git a/unittests/test_python_version.py b/unittests/test_python_version.py
index 09c18eb..09d6f35 100644
--- a/unittests/test_python_version.py
+++ b/unittests/test_python_version.py
@@ -19,7 +19,6 @@ def test_available_python_versions_exist_in_nixpkgs(
 @pytest.mark.parametrize(
     "version_string, expected_python_version",
     [
-        ("3.5", PythonVersion.python35),
         ("3.6", PythonVersion.python36),
         ("3.7", PythonVersion.python37),
         ("3.8", PythonVersion.python38),
diff --git a/unittests/test_requirement.py b/unittests/test_requirement.py
index c8c00e0..e7addb1 100644
--- a/unittests/test_requirement.py
+++ b/unittests/test_requirement.py
@@ -3,7 +3,6 @@
 import pytest
 
 from pypi2nix.package_source import GitSource
-from pypi2nix.package_source import HgSource
 from pypi2nix.package_source import PathSource
 from pypi2nix.requirement_parser import ParsingFailed
 from pypi2nix.requirement_parser import RequirementParser
@@ -118,24 +117,6 @@ def test_that_applies_to_target_works_with_in_keyword(
     assert requirement.applies_to_target(current_platform)
 
 
-def test_that_mercurial_source_url_gets_detected(requirement_parser):
-    requirement = requirement_parser.parse(
-        "hg+https://bitbucket.org/tarek/flake8@a209fb6#egg=flake8"
-    )
-    assert isinstance(requirement, UrlRequirement)
-    assert requirement.url() == "hg+https://bitbucket.org/tarek/flake8@a209fb6"
-
-
-@nix
-def test_that_mercurial_source_extracted_is_valid(requirement_parser):
-    requirement = requirement_parser.parse(
-        "hg+https://bitbucket.org/tarek/flake8@a209fb6#egg=flake8"
-    )
-    source = requirement.source()
-    assert isinstance(source, HgSource)
-    source.nix_expression()
-
-
 @nix
 def test_that_git_source_extracted_is_valid(requirement_parser):
     requirement = requirement_parser.parse(