add typing.ContextManager for 3.6+ only

[JelleZijlstra]

This fixes the easier part of #655.

Would it make sense to add a generic typing.ContextManager that exists in any Python version?

[gvanrossum]

[Ahem, let me try that again.]

Would it make sense to add a generic typing.ContextManager that exists in any Python version?

Maybe, but it'll take a long time until everyone has it (esp. Python 3.5 users). I do think there's some room for improvement, esp. since the stdlib name is apparently AbstractContextManager. But let's merge this PR without waiting for that.

[gvanrossum]

Don't merge yet, there may be a problem with this and our internal codebase.

[gvanrossum]

Ironically the issues I am seeing are due to Python 2 code importing ContextLib (inside if MYPY). Let me see what I can do about it.

[gvanrossum]

Sorry, I'm still waiting for a resolution on our end. This affects a fair amount of code.

[JelleZijlstra]

If it's easier to move things forward, I'd be happy to keep contextlib.ContextManager in typeshed for now, although we should eventually remove it.

I imagine it's going to be helpful for use cases like yours if typing.ContextManager existed for Python < 3.6, so it could be used as an ABC. I'll propose that on the typing tracker.

[gvanrossum]

OK, let's close this and keep typing.ContextManager in all versions, per python/typing#422.

[JelleZijlstra]

We'll still need some changes here since currently there is no ContextManager in typing.pyi. I'll reopen this and make the appropriate changes once the typing PR is resolved.

[gvanrossum]

Oh wait, you're right. This PR removes it from contextlib. We may still want that but first we want to add it to typing, so I can switch our usages to the latter.

FWIW the usages seem to be of the form

if MYPY:
    from contextlib import ContextManager
.
.
.
def foo(a):
    # type: (...) -> ContextManager[...]
    ...