packaging is the standard tool for dealing with versions and
requirements. This allows us to easily support all PEP 508 specs.
packaging also has good error messages, so I don't create our own here.
This lets us get rid of our handrolled half parser and sanity checks.
I kept the whitespace consistency check, since that's just a formatting
lint and isn't testing the correctness of parsing (which we now
definitely do correctly).
- mypy_test and pyright no longer just test the stubs, they now also test other parts of typeshed as well.
- pytype_test.py can now be run on 3.10, meaning the whole test suite can now also be run on 3.10.
- Various test scripts now have from `__future__ import annotations`, meaning they can now only be run on 3.7+.
- Clean up the description of pyright_test.py, which had a slightly confusing wording.
- Also fix the `--dry-run` config option in mypy_test.py, which I accidentally broke in #7746
Move and consolidate venv setup and running isort/black/flake8 into
separate sections and link those sections from the ToC. Also extend
those sections slightly.
Move the tests section into a separate file.
Make venv name in pre-commit match name in CONTRIBUTING.
