[issue43848] gzip.py: explain optional argument mtime

[Joachim]

Change by Joachim <j.wuttke at fz-juelich.de>:


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keywords: +patch
pull_requests: +24142
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/25410

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43848>
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