[issue37129] Add os.RWF_APPEND flag for os.pwritev

[YoSTEALTH]

Change by YoSTEALTH <ritesh at stealthcentral.com>:


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pull_requests: +19604
pull_request: https://github.com/python/cpython/pull/20336

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37129>
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