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

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan <tir.karthi at gmail.com>:


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nosy: +rhettinger, steven.daprano, taleinat

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