[docs] [issue15204] Deprecate the 'U' open mode

[Roundup Robot]

Roundup Robot added the comment:

New changeset 70bd6f7e013b by Serhiy Storchaka in branch 'default':
Issue #15204: Deprecated the 'U' mode in file-like objects.
http://hg.python.org/cpython/rev/70bd6f7e013b

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nosy: +python-dev

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