Annotate known magic-method return types (#9131)

stubs/invoke/invoke/config.pyi

@@ -10,7 +10,7 @@ class DataProxy:
     def __iter__(self): ...
     def __eq__(self, other): ...
     __hash__: ClassVar[None]  # type: ignore[assignment]
-    def __len__(self): ...
+    def __len__(self) -> int: ...
     def __setitem__(self, key, value) -> None: ...
     def __getitem__(self, key): ...
     def __contains__(self, key): ...