Add pre-commit config for pyflakes

[honno]

I keep on forgetting to run pyflakes before pushing, so I thought I'd use pre-commit. Here's a minimal config for the tool. As there doesn't seem to be a pre-commit hook for pyflakes itself, I just use flake8 and run only the pyflake-related checks via flake8 --select F. I've also updated the linting workflow to use pre-commit, to keep consistency (although I doubt flake8 versions will change the pyflake linting behaviour).

It is "another thing", so I understand if you're not interested in merging. For the future I was thinking it might be nice to create a custom pre-commit hook that ensured any staged changes to ./generate_stubs.py would trigger a check to see the related files have been updated.

[asmeurer]

I'd prefer to keep this unpinned. Whenever pyflakes is updated it may create new failures, but those are also things we'd want to know about. Also we are going to want to add 3.10 testing pretty soon, and I don't know if this versions supports it yet.

[honno]

Ah, I had just used the latest release tag (didn't think to unpin it, makes sense). The 4.0 release was tested for py3.10 it looks like at least.

[asmeurer]

Is there any way to remove this? I tried removing it but that causes it to fail.

[asmeurer]

This is fine. It seems optional.

We can try generate stubs too. Sometimes, I do manually update the stubs from a PR to the API repo so that I don't have to wait for it to get merged. Also the API repo is going to change to rst pretty soon, which will break generate stubs until it gets updated.