[issue42501] Improve error messages for argparse choices using enum

[Raymond Hettinger]

Change by Raymond Hettinger <raymond.hettinger at gmail.com>:


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keywords: +patch
pull_requests: +22444
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/23563

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42501>
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