[issue43029] unittest: Add assertUniqeIn

[Denis Roussel]

Change by Denis Roussel <roussel2nis at gmail.com>:


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keywords: +patch
pull_requests: +23154
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/24336

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43029>
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