[issue31208] Simplify `low_fds_to_close` in subprocess.py

[TaoQingyun]

Changes by TaoQingyun <845767657 at qq.com>:


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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue31208>
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