[issue30014] Speedup DefaultSelectors.modify() by 2x

[Giampaolo Rodola']

Changes by Giampaolo Rodola' <g.rodola at gmail.com>:


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versions: +Python 3.7
Added file: http://bugs.python.org/file46788/bench.py

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30014>
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