add __dict__ attribute to class instance

stdlib/3/builtins.pyi

@@ -28,6 +28,7 @@ property = object()
 class object:
     __doc__ = ...  # type: str
     __class__ = ...  # type: type
+    __dict__ = ...  # type: Dict[str, Any]
 
     def __init__(self) -> None: ...
     def __new__(cls) -> Any: ...