[PEP 695] Allow covariance with attribute that has "_" name prefix

[JukkaL]

Fix this conformance test:

class ShouldBeCovariant5[T]:
    def __init__(self, x: T) -> None:
        self._x = x

    @property
    def x(self) -> T:
        return self._x

vo5_1: ShouldBeCovariant5[float] = ShouldBeCovariant5[int](1)  # OK
vo5_2: ShouldBeCovariant5[int] = ShouldBeCovariant5[float](1)  # E

My fix is to treat such attributes as not settable when inferring variance.

Link:
https://github.com/python/typing/blob/main/conformance/tests/generics_variance_inference.py#L79

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅