I assume you've verified that this change resolves your issue?

Yes, I have tested it and mypy no longer complains.

It could be also possible to add tests to check the compatibility in the future.
Or even check mypy for the whole repo.

If it's easy to set up a mypy check in tox, feel free to propose it. I would be happy to include it in this PR or to merge this and have the mypy checks in a future PR.

Im a bit busy at the moment but would be happy to give it a try.
Should only files which are created be tested or the full repo?

If you check tox, we style check two generated files. Probably makes the most sense to test just those.

It seems that setuptools is lazy with type hints, so the verioneer.py file causes issues.
However, most projects will only check the src directory and not the root directory, so only checking the git_version.py is fine.

Should work like this, if wanted I can already start a Release 0.21 section in the NEWS.md and add this

Any idea why the setup fails?

mypy does not seem to install on pypy (python/typed_ast#111). I don't know if Tox will allow us to skip certain dependencies and test steps. Feel free to look into it, or I can try to find time this weekend.