Fixed constant_time_compare on Python 2.7.7

Python 2.7.7 includes compare_digest in the hmac module, but it requires
both arguments to have the same type. This is usually not a problem on
Python 3 since everything is text, but we have mixed unicode and str on
Python 2 -- hence make sure everything is bytes before feeding it into
compare_digest.

@dstuff, @alex, @PaulMcMillan -- review please, I'd like to get this into Django 1.7