build: unbreak dependency tracking, doc builds and multiline tables

[jktjkt]

build: specify dependencies directly in setup.cfg

In tox v4, "reuse of environments" was disabled. This is then later explained to refer to exactly that thing which we were using for inheriting the dependencies from the top-level testenv all the way to the docs build. That's why the docs build in GitHub CI started failing.

IMHO, The Correct Way™ of specifying what dependencies are used for which feature are the so-called "extra dependencies". Once they are in place, it is be possible to install gnpy via, e.g., pip install oopt-gnpy[docs]. However, this process is (as far as I can tell) incompatible with requirements.txt; all my attempts at using the standard dependency syntax in that file have failed for me.

So, in order to make this happen, let's move all the dependencies from a more-or-less ad-hoc collection of files to this declarative approach right in setup.cfg. That way, the deps are listed on a single place, in a declarative manner, and as a result, the installation is now a trivial pip oneliner.

As a result, one can also remove that duplication of dependencies in docs requirements.

Fixes: 47c8962 fix docs requirements

docs: try to unbreak the readthedocs.io build

It was failing with a message:

Config validation error in build.os. Value os not found.

Apparently, the v2 config file is mandatory, so let's do that.

docs: use the default theme on ReadTheDocs.org as well

Historically, we've been using the RTD theme on the RTD site which hosts
the docs for us, and a Sphinx-default, "Alabaster" theme for other docs
builds. Doing that however started failing:

Traceback (most recent call last):
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/html/__init__.py", line 1096, in handle_page
 output = self.templates.render(templatename, ctx)
	  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/readthedocs_ext/readthedocs.py", line 181, in rtd_render
 content = old_render(template, render_context)
	   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/jinja2glue.py", line 194, in render
 return self.environment.get_template(template).render(context)
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/jinja2/environment.py", line 1301, in render
 self.environment.handle_exception()
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/jinja2/environment.py", line 936, in handle_exception
 raise rewrite_traceback_stack(source=source)
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/basic/page.html", line 10, in top-level template code
 {%- extends "layout.html" %}
 ^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/classic/layout.html", line 10, in top-level template code
 {%- extends "basic/layout.html" %}
 ^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/default/../basic/layout.html", line 170, in top-level template code
 {%- block content %}
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/default/../basic/layout.html", line 189, in block 'content'
 {%- block sidebar2 %}{{ sidebar() }}{% endblock %}
 ^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/default/../basic/layout.html", line 189, in block 'sidebar2'
 {%- block sidebar2 %}{{ sidebar() }}{% endblock %}
 ^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/jinja2/sandbox.py", line 393, in call
 return __context.call(__obj, *args, **kwargs)
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/jinja2/runtime.py", line 777, in _invoke
 rv = self._func(*arguments)
      ^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/themes/default/../basic/layout.html", line 63, in template
 {%- include sidebartemplate %}
 ^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/jinja2glue.py", line 215, in get_source
 raise TemplateNotFound(template)
jinja2.exceptions.TemplateNotFound: about.html

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/cmd/build.py", line 281, in build_main
 app.build(args.force_all, args.filenames)
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/application.py", line 347, in build
 self.builder.build_update()
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 310, in build_update
 self.build(to_build,
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 376, in build
 self.write(docnames, list(updated_docnames), method)
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 571, in write
 self._write_serial(sorted(docnames))
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 581, in _write_serial
 self.write_doc(docname, doctree)
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/html/__init__.py", line 672, in write_doc
 self.handle_page(docname, ctx, event_arg=doctree)
File "/home/docs/checkouts/readthedocs.org/user_builds/gnpy/envs/499/lib/python3.12/site-packages/sphinx/builders/html/__init__.py", line 1103, in handle_page
 raise ThemeError(__("An error happened in rendering the page %s.\nReason: %r") %
sphinx.errors.ThemeError: An error happened in rendering the page about-project.
Reason: TemplateNotFound('about.html')

Theme error:
An error happened in rendering the page about-project.
Reason: TemplateNotFound('about.html')

I have no clue what that means because we have never requested this
about.html, nor do we reference that file from anywhere. Chances are
that it's "just" some version pinning/compatibility issue, but hey --
why mess with that when there's a perfectly good default theme that
we're using for other purposes already.

As a side effect, this also solves that long-standing issue that Esther
reported where the tables have overly long lines. Apparently, it's a
theme-specific misfeature (readthedocs/sphinx_rtd_theme/#117), and the
Alabaster one doesn't suffer from that.

All hail alabaster!

[EstherLerouzic]

bravo !