Add typeguards to __contains__ methods

Author: George-Ogden

stdlib/typing.pyi

@@ -665,7 +665,7 @@ class Sequence(Reversible[_T_co], Collection[_T_co]):
     # Mixin methods
     def index(self, value: Any, start: int = 0, stop: int = ..., /) -> int: ...
     def count(self, value: Any, /) -> int: ...
-    def __contains__(self, value: object, /) -> bool: ...
+    def __contains__(self, value: object, /) -> TypeGuard[_T_co]: ...
     def __iter__(self) -> Iterator[_T_co]: ...
     def __reversed__(self) -> Iterator[_T_co]: ...
 
@@ -791,7 +791,7 @@ class Mapping(Collection[_KT], Generic[_KT, _VT_co]):
     def items(self) -> ItemsView[_KT, _VT_co]: ...
     def keys(self) -> KeysView[_KT]: ...
     def values(self) -> ValuesView[_VT_co]: ...
-    def __contains__(self, key: object, /) -> bool: ...
+    def __contains__(self, key: object, /) -> TypeGuard[_KT]: ...
     def __eq__(self, other: object, /) -> bool: ...
 
 class MutableMapping(Mapping[_KT, _VT]):