Remove 'U' flag in Python 3

[tjni]

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Proposed changes

This mode has been removed in Python 3.11. Per the migration guide:

open(), io.open(), codecs.open() and fileinput.FileInput no longer accept 'U' (“universal newline”) in the file mode. In Python 3, “universal newline” mode is used by default whenever a file is opened in text mode, and the 'U' flag has been deprecated since Python 3.3. The newline parameter to these functions controls how universal newlines work. (Contributed by Victor Stinner in bpo-37330.)

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