[3.14] gh-134970: Fix exception message in argparse module (GH-134971) (GH-134991)

Fix the "unknown action" exception in argparse.ArgumentParser.add_argument_group()
to correctly replace the action class.
(cherry picked from commit 965c48056633d3f4b41520c8cd07f0275f00fb4c)

Co-authored-by: Michał Górny <mgorny@gentoo.org>