[issue40331] Increase test coverage for the statistics module

[Steven D'Aprano]

Steven D'Aprano <steve+python at pearwood.info> added the comment:

*blink*

How did I miss this entire thing? Where was the code review?

Thanks Tal for shepherding this through, and thanks Tzanetos for caring 
about the unit tests.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40331>
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