[issue16399] argparse: append action with default list adds to list instead of overriding

[hai shi]

Change by hai shi <shihai1991 at 126.com>:


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pull_requests: +17226
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/17793

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue16399>
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