Use typing.ContextManager to fix multiprocessing context managers

[OddBloke]

No description provided.

[JelleZijlstra]

Can you clarify what this is "fixing"? Is it about passing these objects to functions that expect a ContextManager?

[OddBloke]

I actually started just looking at missing types (from the warnings in #1526), and that's what I was referring to fixing. Using ContextManager seemed like the best way to address the lack of types on __enter__ and __exit__, and being able to use them as generic context managers is a nice side effect.

[OddBloke]

When looking at #1559, I've modified this code further, so I'll close this out and open a fresh PR with those changes.