[issue39354] collections.UserString format and format_map return a string

[bizywizy]

Change by bizywizy <bizywiz at gmail.com>:


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keywords: +patch
pull_requests: +17421
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/18026

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39354>
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