[issue28919] Simplify `_copy_func_details` in unittest.mock

[Berker Peksag]

New submission from Berker Peksag:

Thanks for the patch. I only did a quick review, but it looks good to me.

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nosy: +berker.peksag, michael.foord
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28919>
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