bpo-42547: fix add_argument(metavar=<tuple>, nargs='+') for positional arguments

[m-khvoinitsky]

https://bugs.python.org/issue42547

• Issue: argparse: add_argument(... nargs='+', metavar=<tuple>) does not work with positional arguments #86713

[the-knights-who-say-ni]

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[ZackerySpytz]

Sorry, but there really needs to be a unit test for this change.

[ZackerySpytz]

Please add more details and use full sentences. Code snippets should be enclosed in double backquotes.

Please replace "argparse" with

:mod:`argparse`

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[serhiy-storchaka]

Thank you for your PR, @m-khvoinitsky, but it was not be merged dues to lack of tests. And tests could show that these changes are not enough.

#124782 is a more complete solution for this issue.