[3.12] gh-61011: Fix inheritance of nested mutually exclusive groups in argparse (GH-125210) (GH-125309)

Previously, all nested mutually exclusive groups lost their connection
to the group containing them and were displayed as belonging directly
to the parser.

(cherry picked from commit 18c74497681e0107d7cde53e63ea42feb38f2176)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Danica J. Sutherland <djsutherland@users.noreply.github.com>