Add missing type hint for AbstractSet._hash(). (#7153)

stdlib/typing.pyi

@@ -371,6 +371,7 @@ class MutableSequence(Sequence[_T], Generic[_T]):
 class AbstractSet(Collection[_T_co], Generic[_T_co]):
     @abstractmethod
     def __contains__(self, x: object) -> bool: ...
+    def _hash(self) -> int: ...
     # Mixin methods
     def __le__(self, s: AbstractSet[Any]) -> bool: ...
     def __lt__(self, s: AbstractSet[Any]) -> bool: ...