os._AddedDllDirectory: add __init__ (#4498)

stdlib/3/os/__init__.pyi

@@ -840,6 +840,7 @@ if sys.version_info >= (3, 8):
     if sys.platform == "win32":
         class _AddedDllDirectory:
             path: Optional[str]
+            def __init__(self, path: Optional[str], cookie: _T, remove_dll_directory: Callable[[_T], Any]) -> None: ...
             def close(self) -> None: ...
             def __enter__(self: _T) -> _T: ...
             def __exit__(self, *args: Any) -> None: ...