[issue41244] Change to use str.join() instead of += when concatenating string

[Wansoo Kim]

Change by Wansoo Kim <rladhkstn8 at gmail.com>:


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keywords: +patch
pull_requests: +20546
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/21398

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41244>
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