add object.__dir__ in Python 3 (#1532)

In Python 3 (but not Python 2), `object().__dir__()` works and returns a list of strings.

This is relevant when implementing a custom `__dir__` that invokes `super().__dir__()`.

stdlib/3/builtins.pyi

@@ -50,6 +50,7 @@ class object:
     def __sizeof__(self) -> int: ...
     def __reduce__(self) -> tuple: ...
     def __reduce_ex__(self, protocol: int) -> tuple: ...
+    def __dir__(self) -> List[str]: ...
 
     if sys.version_info >= (3, 6):
         def __init_subclass__(cls) -> None: ...