unsafe variance errors shouldn't propagate down

Same thing with unsafe variance in protocol inheritance:
```py
from typing import Protocol, _T_co as T_co

class A(Protocol[T_co]): # type: ignore
def foo(self, t: T_co) -> None: ... # type: ignore

class B(A[T_co], Protocol): # ERROR! using covariant type variable where invariant one expected!!!
def bar(self) -> T_co: ...
```

[playground](https://mypy-play.net/?gist=f763a37b161f534caa8a76e67ff24757)

Start with the minimal Python reproduction in the issue and compare its diagnostics with the linked mypy-play playground. Trace protocol inheritance and unsafe-variance checking to determine why the error propagates into B; done means the shown inheritance pattern no longer reports the unwanted error while preserving the relevant variance checks.

Written by the indexing model from the issue text.