Fixes mypy crash on protocol with contravariant var (#11135)
Closes #11020
* Fixes mypy crash on protocol with contravariant var
* Fixes how `infer_constraints_from_protocol_members` works
Here's what was wrong:
1. When checking if some type is a subtype of a protocol, we iterate over all its members
2. When we are inside `is_protocol_implementation` we set `.assuming` or `.assuming_proper`
attributes and continue to check the other members
3. While checking methods with annotated `self` type with a protocol itself, we recurse into
`is_protocol_implementation` once again
4. It always returns `True` in this case, because `assuming` is set in the context above
5. Because `is_protocol_implementation` is set to `True`, we dive into
`infer_constraints_from_protocol_members` with a type, which is not really a protocol
implementation
6. Sanity check breaks!
Solution: return empty constraints instead of raising `AssertionError`
