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

[miss-islington]

Change by miss-islington <mariatta.wijaya+miss-islington at gmail.com>:


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pull_requests: +16508
pull_request: https://github.com/python/cpython/pull/16982

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