Pytype should support `__all__ +=` in stubs

[rchen152]

See python/typeshed#7865. Supporting += on __all__ allows a ton of code duplication to be avoided.

[rchen152]

cc @martindemello since you looked at __all__ in stubs recently - is this something that would be easy to support?

[martindemello]

should be a bit easier now that we parse __all__ as a literal constant. i'll take a look at it.