Typing spec update for exceptions / context managers #27

I'd like to request that the TC consider adoption of a new (small) chapter in the typing spec that spells out the type checking behaviors for context managers — specifically, how to use the return type of __exit__ to determine whether a context manager suppresses exceptions.

Links to PR & Discussion
The PR can be found here. It incorporates feedback from PR reviewers.

The discussion can be found here.

TC Sign-off

• Eric
• Guido
• Jelle
• Rebecca
• Shantanu

Current Type Checker Behaviors
The specified behavior is consistent with the current behavior of mypy and pyright. It is also consistent with assumptions that are baked into typeshed stubs. I haven't done detailed testing on pyre or pytype, but given that they leverage typeshed stubs, they likely conform.