Infer empty list without annotation for __slots__ and module __all__

[sterliakov]

Fixes #10870, fixes #10103.

This adds a fake Iterable[str] context when checking the following:

• __all__ = [] at top level
• __slots__ = [] at class level (also works for sets but not for dicts)

Additionally, this fixes a bug with __slots__ being mistakenly checked in other contexts (at top level or in function bodies), so e.g. the following is now accepted:

def foo() -> None:
    __slots__ = 1

[sobolevn]

Thank you!

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅