[issue36824] Refactor str tests to reflect that str and unicode are merged in Python 3

[Daniel Fortunov]

Change by Daniel Fortunov <pythonbugs at danielfortunov.com>:


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keywords: +patch
pull_requests: +13088
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36824>
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