mypy integration

[iddan]

Collections should implement mypy Mapping generic to be properly typed

[antonagestam]

I'd be happy to get the ball rolling if PR's will be considered for this :)

[iddan]

You just made my day 😄

[antonagestam]

It turned out that this what a bit more complicated than I originally thought. For types such as Map[str, int] to not break at runtime the Map class needs to be subscribable. You normally achieve that by making it inherit from typing.Mapping (or typing.Generic). But since Map is implemented as a C extension this doesn't seem that straight forward. It might be simple to do this in the C implementation though, I'll try and dig further when I get the time.

[antonagestam]

Got some tips in the typing gitter channel: https://gitter.im/python/typing?at=5d25bd59cdb70805c96e239f