KeysView's internal mapping no longer expresses __contains__ #14884

# Suitable for dictionary view objects
class Viewable(Protocol[_T_co]):
    def __len__(self) -> int: ...
    def __iter__(self) -> Iterator[_T_co]: ...

class KeysView(MappingView, Set):

    __slots__ = ()

    @classmethod
    def _from_iterable(cls, it):
        return set(it)

    def __contains__(self, key):
        return key in self._mapping

    def __iter__(self):
        yield from self._mapping