bpo-42980: Add GnuStyleLongOptionsHelpFormatter

[ouillie]

Add a new help formatter for argparse that prints long options
in a style consistent with GNU getopt long options.

https://bugs.python.org/issue42980

[the-knights-who-say-ni]

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[ouillie]

Note that I added .. versionadded:: 3.10 in the documentation for this class, since I assume it will not be merged until that release at the earliest. Not sure if that's realistic or not.

[github-actions]

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