Allow incompatible overrides on `__slots__`

[JelleZijlstra]

This should be allowed:

class A:
    __slots__ = ("a",)

    def __init__(self) -> None:
        self.a = 3

class B(A):
    __slots__ = ("b",)

    def __init__(self) -> None:
        super().__init__()
        self.b = 4

But currently produces incompatible_override. Special-case __slots__ to allow this.