[issue38474] digit check logic can be replaced by Py_ISDIGIT on prepare_s

[Dong-hee Na]

Change by Dong-hee Na <donghee.na92 at gmail.com>:


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versions: +Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38474>
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