[issue37804] Remove Thread.isAlive in Python 3.9

[Dong-hee Na]

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


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keywords: +patch
pull_requests: +14949
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/15225

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<https://bugs.python.org/issue37804>
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