CLN: fix py2to3 issues in categorical.py
how is it possible that these passed import? They should fail in Python 3.2 as syntax errors!
 they are! but I think that original PR must have been before your 2-to-3 changes so it look 'passed'

when I merged them in it passed in 2.7 ok (as the u"") syntax is ok in 2.7
