[3.12] gh-63143: Fix parsing mutually exclusive arguments in argparse (GH-124307) (GH-124419)

Arguments with the value identical to the default value (e.g. booleans,
small integers, empty or 1-character strings) are no longer considered
"not present".
(cherry picked from commit 3094cd17b0e5ba69309c54964744c797a70aa11b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>