[issue43160] argparse: add extend_const action

[Tony Lykke]

Change by Tony Lykke <hi at tonylykke.com>:


----------
keywords: +patch
pull_requests: +23269
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/24478

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43160>
_______________________________________