Use contravariant type variable rather than object in Container (#15320)

Author: randolf-scholz

stdlib/typing.pyi

@@ -640,14 +640,17 @@ class AsyncGenerator(AsyncIterator[_YieldT_co], Protocol[_YieldT_co, _SendT_cont
     ) -> Coroutine[Any, Any, _YieldT_co]: ...
     def aclose(self) -> Coroutine[Any, Any, None]: ...
 
+_ContainerT_contra = TypeVar("_ContainerT_contra", contravariant=True, default=Any)
+
 @runtime_checkable
-class Container(Protocol[_T_co]):
+class Container(Protocol[_ContainerT_contra]):
     # This is generic more on vibes than anything else
     @abstractmethod
-    def __contains__(self, x: object, /) -> bool: ...
+    def __contains__(self, x: _ContainerT_contra, /) -> bool: ...
 
 @runtime_checkable
-class Collection(Iterable[_T_co], Container[_T_co], Protocol[_T_co]):
+class Collection(Iterable[_T_co], Container[Any], Protocol[_T_co]):
+    # Note: need to use Container[Any] instead of Container[_T_co] to ensure covariance.
     # Implement Sized (but don't have it as a base class).
     @abstractmethod
     def __len__(self) -> int: ...