argparse stub fix: values parameter of Action can also be str and None (#388)

* values can be a str, too

* values should probably use Sequence

stdlib/2and3/argparse.pyi

@@ -109,7 +109,8 @@ class Action:
                  help: Optional[str] = ...,
                  metavar: Union[str, Tuple[str, ...]] = ...) -> None: ...
     def __call__(self, parser: ArgumentParser, namespace: Namespace,
-                 values: List[Any], option_string: str = ...) -> None: ...
+                 values: Union[str, Sequence[Any], None],
+                 option_string: str = ...) -> None: ...
 
 class Namespace:
     def __getattr__(self, name: str) -> Any: ...