[issue39094] Add a default to statistics.mean and related functions

[Yoni Lavi]

Change by Yoni Lavi <yoni at lavi.fm>:


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keywords: +patch
pull_requests: +17124
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/17657

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39094>
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