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

[Vinay Sajip]

Change by Vinay Sajip <vinay_sajip at yahoo.co.uk>:


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pull_requests:  -11443

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