Typeshed cherry-pick: Add __eq__ to types.MappingProxyType (#9580) (#14507)

This fixes a false positive when using `--strict-equality`.

See
python/typeshed@d5b88c5
for context.

Author: JukkaL

mypy/typeshed/stdlib/types.pyi

@@ -310,6 +310,7 @@ class MappingProxyType(Mapping[_KT, _VT_co], Generic[_KT, _VT_co]):
     def __getitem__(self, __key: _KT) -> _VT_co: ...
     def __iter__(self) -> Iterator[_KT]: ...
     def __len__(self) -> int: ...
+    def __eq__(self, other: object) -> bool: ...
     def copy(self) -> dict[_KT, _VT_co]: ...
     def keys(self) -> KeysView[_KT]: ...
     def values(self) -> ValuesView[_VT_co]: ...