add contextlib.chdir

[graingert]

see python/cpython#28271

Is it too early to start adding 3.11 types to typeshed? I'm looking to add it here so I can backport the types cleanly into contextlib2

[Akuli]

It might be a bit early because 3.11 can still change, but we can always change this later. I would like to know what other maintainers think.

[srittau]

I'm fine with adding 3.11 annotations, especially ones that are not likely to change. Less chance to forget them later.

[github-actions]

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