Fix variance inference in this fragment from a typing conformance test:
```
class ClassA[T1, T2, T3](list[T1]):
    def method1(self, a: T2) -> None:
        ...

    def method2(self) -> T3:
        ...
```

Previously T2 was incorrectly inferred as invariant due to `list` having
methods that return `Self`. Be more flexible with return types to allow
inferring contravariance for type variables even if there are `Self` return
types, in particular.

We could probably make this even more lenient, but after thinking about
this for a while, I wasn't sure what the most general rule would be, so
I decided to just make a tweak to support the likely most common use case
(which is probably actually not that common either).

Link to conformance test:
https://github.com/python/typing/blob/main/conformance/tests/generics_variance_inference.py#L15C1-L20C12