[docs] [issue35781] `logger.warn` method is used in "Logging HOWTO" documentation though `logger.warn` method is deprecated in Python 3.7

[Yuuji KAWAMATSU]

Change by Yuuji KAWAMATSU <kawamatsu_yuuji at kurusugawa.jp>:


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keywords: +patch, patch, patch
pull_requests: +11442, 11443, 11444
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35781>
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