black: enable skip_magic_trailing_comma

[srittau]

Let's wait for the hotfixed black version, per psf/black#2141.

[github-actions]

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

[github-actions]

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

[JelleZijlstra]

I think it's fine to merge this now even without the release. CI will work fine because the cache gets wiped every time, and the cache is versioned by Black version so contributors are unlikely to have a bad cache locally for the new Black version. We're likely to have a new release out by tomorrow though.

[JelleZijlstra]

New release just came out, I'll pin black to the new version and then land this PR.

[JelleZijlstra]

Looks like there's a permission issue with the mypy-primer comment workflow: Error: Unhandled error: HttpError: Resource not accessible by integration.