bpo-11874: fix assertion failure in argparse metavar handling (GH-1826)


- bugfix and test for fragile metavar handling in argparse (see
  bpo-24089, bpo-14046, bpo-25058, bpo-11874)
- also fixes some incorrect tests that did not make 1-element tuples correctly
(cherry picked from commit 66f02aa32f1e4adb9f24cf186f8c495399d5ce9b)

Co-authored-by: wim glenn <wim.glenn@gmail.com>
3 files changed