Default to pyright, not mypy

We can't see any reason to have two type checkers: pyright (part of
pylance) and also mypy. So we've defaulted to "pyright" and removed
"mypy".

This way, if we fix the errors in VSCode, we don't expect to see them
when we run `make types` or `make check`. Also, they should run fine in
Github Actions too.

There's good reasons to use pyright over mypy [1], including:

- Speed. pyright is 3x - 5x faster than mypy when checking large code
  bases. Not that this should be a reason in and of itself.

- Consistency. pyright generally tries to adhere to mypy's behaviour
  unless there is a compelling justification for deviating.

- Ease. pylance includes pyright already in the VS Code extension. So
  not much thinking required to do type checking.

- Dogma. Our main reason is that pyright performs type checking for all code
  regardless of whether it contains type annotations. By default, mypy
  skills all functions or methods that do not have type annotations. We
  believe that type hints are "hints", and should not be enforced. We
  wish to be productive not dogmatic.

[1] https://github.com/microsoft/pyright/blob/main/docs/mypy-comparison.md

Makefile

@@ -25,7 +25,7 @@ format:
 
 .PHONY: types
 types:
-	mypy .
+	pyright .
 
 .PHONY: test
 test:

pyrightconfig.json

@@ -0,0 +1,8 @@
+{
+  "reportIncompatibleMethodOverride": "error",
+  "reportMissingParameterType": "none",
+  "reportMissingTypeStubs": "none",
+  "typeCheckingMode": "basic",
+  "venv": ".venv",
+  "venvPath": "."
+}