Run pycln as a pre-commit hook in CI

[AlexWaygood]

pycln is a pre-commit hook to auto-remove unused import statements. The tool is used by several high-profile open-source projects such as Pyodide, Poetry and Rich. It is fully compatible with .pyi files as well as .py files.

[AlexWaygood]

Here's pycln successfully auto-removing some unused imports that pyright didn't pick up on: 8d0d6d3

[srittau]

LGTM, but I would like to hear feedback of other maintainers.

[hauntsaninja]

I'm fine with this. Not familiar with pycln, but it looks like it uses libcst, which is promising (unlike isort, which was plagued with bugs for a long time).

[Akuli]

Have you tried running pycln on all of our existing stubs? Does it delete anything? If it does, a file like that would be quite annoying to contribute to.

[AlexWaygood]

Have you tried running pycln on all of our existing stubs? Does it delete anything? If it does, a file like that would be quite annoying to contribute to.

Yes, I ran it on the whole of typeshed before filing this PR.

It made changes to aiohttp, where it picked up a genuine bug (an import wasn't being re-exported when it should have been). I fixed that in the first commit in this PR: b7e0b2d.

It also identified unused imports in stdlib/tokenize.pyi and stdlib/unittest/loader.pyi. pycln auto-fixed these in the third commit in this PR: 8d0d6d3.

So if this PR is merged, the entirety of typeshed should be pycln- compatible.

[github-actions]

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