Note that round-trip also fails in weeks 52-53.  For example,

>>> datetime.strptime('2014 53 6', '%Y %W %w')
datetime.datetime(2015, 1, 10, 0, 0)
>>> datetime.strptime('2014 53 6', '%Y %W %w').strftime('%Y %W %w')
'2015 01 6'

If we decide to make "2015 0 0" invalid for format '%Y %W %w', we should probably invalidate the entire week 53 in 2014 for consistency.  However, I don't think there are any C implementations that would have a problem with such dates.

Overall, I am inclined to accept Jim's solution for 3.5, but I am not sure about the maintenance branches.