Add missing __eq__ to Mapping (#10297)

Co-authored-by: Alex Waygood <[email protected]>

Author: Dreamsorcerer

stdlib/typing.pyi

@@ -631,6 +631,7 @@ class Mapping(Collection[_KT], Generic[_KT, _VT_co]):
     def keys(self) -> KeysView[_KT]: ...
     def values(self) -> ValuesView[_VT_co]: ...
     def __contains__(self, __key: object) -> bool: ...
+    def __eq__(self, __other: object) -> bool: ...
 
 class MutableMapping(Mapping[_KT, _VT], Generic[_KT, _VT]):
     @abstractmethod