[issue23776] Don't use assert for checking arguments in pprint

[Berker Peksag]

Berker Peksag added the comment:

Looks good. Two things:

* I'd apply this only to the default branch. Changing exceptions from AssertionError to ValueError in bugfix releases may break third party code. I couldn't find a similar bug report in the tracker, so it's probably not worth to take that risk.
* We should audit the standard library for similar issues

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assignee:  -> serhiy.storchaka
nosy: +berker.peksag
stage: patch review -> commit review

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23776>
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