[protobuf] `UnknownFieldSet.__getitem__ takes only positional arguments (#15599)

Author: srittau

stubs/protobuf/google/protobuf/unknown_fields.pyi

@@ -5,5 +5,5 @@ from google.protobuf.message import Message
 @final
 class UnknownFieldSet:
     def __new__(cls, msg: Message) -> UnknownFieldSet: ...  # noqa: Y034
-    def __getitem__(self, index: int) -> Any: ...  # Any: internal unknown field object
+    def __getitem__(self, index: int, /) -> Any: ...  # Any: internal unknown field object
     def __len__(self) -> int: ...