`KeysView`'s internal mapping no longer expresses `__contains__`

[iutlu]

With this PR #14574, the previous Mapping annotation for the internal mapping attribute of KeysView seems to have been changed to a Viewable protocol:

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

I was comparing this against CPython's collections.abc

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

and it appears that self._mapping has now lost the __contains__ (which was previously guaranteed by Mapping). Wanted to raise it here.

[iutlu]

Apologies, __iter__ is sufficient for the in check -- I'd missed that.