backend_qt5.py Don't use six.u

This code already does from __future__ import unicode_literal.
Thus '+' will be a unicode string even in python 2 and there is no
need for converting it into a unicode string. Infact this results in
a TypeError because six.u assumes the input string to be ASCII only.
See http://pythonhosted.org/six/#six.u
Merge pull request #3357 from jenshnielsen/qt4_unicode

backend_qt5.py Don't use six.u