+1 for the general idea of deprecating and eventually removing the "U"
modes.

But I agree with David, that it doesn't make sense to have separate steps
for 3.5 and 3.6/4.0. If you make the code raise an exception when "U" is
used, how is that different from what will happen when you remove the
code for processing it? Surely we want it to eventually be treated just
like any other invalid mode string?