pythongh-134970: Fix exception message in argparse module (pythonGH-134971)

Fix the "unknown action" exception in argparse.ArgumentParser.add_argument_group()
to correctly replace the action class.

Author: mgorny

Lib/argparse.py

@@ -1534,7 +1534,7 @@ def add_argument(self, *args, **kwargs):
         action_name = kwargs.get('action')
         action_class = self._pop_action_class(kwargs)
         if not callable(action_class):
-            raise ValueError('unknown action {action_class!r}')
+            raise ValueError(f'unknown action {action_class!r}')
         action = action_class(**kwargs)
 
         # raise an error if action for positional argument does not

Misc/NEWS.d/next/Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst

@@ -0,0 +1,3 @@
+Fix the "unknown action" exception in
+:meth:`argparse.ArgumentParser.add_argument_group` to correctly replace the
+action class.