ruff

7efc9a61 - [ty] Reject incompatible explicit variance in generic base classes (#25327)

[ty] Reject incompatible explicit variance in generic base classes (#25327) ## Summary Prior to this change, we accepted legacy generic subclasses whose explicitly declared variance was incompatible with a specialized base class. For example: ```python from typing import Generic, TypeVar T = TypeVar("T") T_co = TypeVar("T_co", covariant=True) class Invariant(Generic[T]): ... class Bad(Invariant[T_co]): ... ``` `Bad` claims covariance even though its invariant base requires its type parameter to remain invariant. This brings `aliases_variance.py` and `generics_variance.py` into conformance.