[issue37330] open(): remove 'U' mode, deprecated since Python 3.3

[STINNER Victor]

Change by STINNER Victor <vstinner at python.org>:


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pull_requests: +26559
pull_request: https://github.com/python/cpython/pull/28118

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37330>
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