py2k: accept unicode literals on :func:`backref`, too

py2k: accept unicode literals on :func:`backref`, too

Fixed bug where in Py2K a unicode literal would not be accepted as the
string name of a class or other argument within declarative using
:func:`.backref` on :func:`.relationship`.

amends commit e6f67f48054d906856f879bc1803ea639aa4b670

(cherry picked from commit 58f73d2278393d813c7f39736fc96c5086f18f6d)