[3.12] gh-58282: Fix support of tuple metavar for positional arguments in argparse (GH-124782) (GH-124881)

Previously, formatting help output or error message for positional argument
with a tuple metavar raised exception.

(cherry picked from commit 9b31a2d83fa7cb0fe4d75ce7cf6a2c9ea2ce0728)

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