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

Change by bizywizy : -- keywords: +patch pull_requests: +17421 stage: -> patch review pull_request: https://github.com/python/cpython/pull/18026 ___ Python tracker <https://bugs.python.org/issu

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

New submission from bizywizy : collections.UserString.format and collections.UserString.format_map return a string instaed of UserString. This is quite weird because I expect that the %-syntax and `format` method have to produce the same result. ``` >>> isinstance(UserString(