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

[R. David Murray]

R. David Murray <rdmurray at bitdance.com> added the comment:

Unless there are places where it is actually broken, I don't think there is a good reason to have step 3.5, though.  Just add the deprecation warning and remove it in 4.0.

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nosy: +r.david.murray

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