add pre-commit config

[JelleZijlstra]

Fixes #5552.

Also upgrade Black and isort while I'm at it.

Added some random formatting changes in various files to make sure pre-commit fixes it.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JelleZijlstra]

I guess this makes it also run black on the tests and scripts directories. That seems like a good thing to me, but I can figure out how to turn it off if that's preferred.

[Akuli]

It would be nice to have a comment about how this version must match requirements-tests-py3.txt, ideally in both places. Same for isort.

[JelleZijlstra]

Good point, done

[ewjoachim]

Hey @JelleZijlstra 👋
Now that this is merged, can you clarify the state of the pre-commit script (https://github.com/python/typeshed/blob/master/pre-commit) and the documentation (https://github.com/python/typeshed/blob/master/CONTRIBUTING.md#preparing-the-environment) ? Is it still relevant ? Is it expected that we merge the 2 scripts locally ? Or...?

[hauntsaninja]

Feel free to submit a PR removing the old script. typeshed doesn't really care what you do locally, pre-commit.ci will now automatically fix any formatting issues in PRs.

[Akuli]

As a maintainer, I have never used the local pre-commit script, and I have always considered it deprecated :)