[3.14] gh-142346: Fix usage formatting for mutually exclusive groups in argparse (GH-142381) (GH-142385)

Support groups preceded by positional arguments or followed or intermixed
with other optional arguments. Support empty groups.
(cherry picked from commit 1db9f56bff5bbb0292b131ea8a928612acb7ec16)

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