Typing spec update: syntax and supported constructs for stub files

[rchen152]

Please consider this proposed change to the spec: https://github.com/python/typing/pull/1815.

The PR has already been reviewed by several people (thank you!). There are a few unresolved points on which discussion has stalled or petered out that I'm hoping the council can make a decision on.

• [Minor] Should pass in class and function bodies means the same thing as ..., or should the behavior of pass be undefined? Discussion
• [Minor] Should class-scoped type aliases be supported? Discussion
• [Controversial] What version of Python syntax should be supported? Discussion1 | Discussion2

Discuss thread here (but the discussion is all on the PR).

TC Sign-off

• [x] Carl
• [x] Eric
• [x] Jelle
• [X] Rebecca
• [x] Shantanu

[rchen152]

It seems like we've reached reasonable conclusions on the unresolved points above (in particular, thanks to Jelle for rewriting the Syntax section), so I've added boxes for TC sign-off.