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

[miss-islington]

• 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 66f02aa)

Co-authored-by: wim glenn wim.glenn@gmail.com

https://bugs.python.org/issue11874

[miss-islington]

@wimglenn and @ncoghlan: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@wimglenn and @ncoghlan: Backport status check is done, and it's a failure ❌ .

[ncoghlan]

@Mariatta I'm not entirely clear what happened here... I think my closing the PR in order to restart the CI by reopening it may have confused miss-islington.

[ncoghlan]

#7530 worked the way we'd expect it to work

[Mariatta]

@ncoghlan Yeah that's the tricky part. miss-islington will delete the branch after the PR has been closed, whether merged or not 😄 So re-creating the PR is the way to go, which you already did.