Since mypy now supports __slots__ checking, we can add __slots__ to stubs.
This will allow us to correctly raise Something is not in __slots__ when assigning extra attributes.

Right now there are no __slots__ defined: https://github.com/python/typeshed/search?q=__slots__

If this is something we are going for, there are several things to consider:

1. Adding this feature to stubtest. It can check that __slots__ contents do match. Probably behind a feature flag. We can enable it for stdlib only

2. Should we allow skipping _protected attributes? For example, if some class is defined as

   class A:
      __slots__ = ('_prop',)

   Should we allow this stub?

   class A:
       __slots__ = ()