[argparse] Allow string actions in add_subparsers (#16318)

Author: nightcityblade

stdlib/argparse.pyi

@@ -223,7 +223,7 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         title: str = "subcommands",
         description: str | None = None,
         prog: str | None = None,
-        action: type[Action] = ...,
+        action: str | type[Action] = ...,
         option_string: str = ...,
         dest: str | None = None,
         required: bool = False,
@@ -238,7 +238,7 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         description: str | None = None,
         prog: str | None = None,
         parser_class: type[_ArgumentParserT],
-        action: type[Action] = ...,
+        action: str | type[Action] = ...,
         option_string: str = ...,
         dest: str | None = None,
         required: bool = False,