Remove unicode literals.

Lets start with this
Part of issue https://github.com/edx/diff-cover/issues/94

Based on django's docs we should just need to remove the u's. Though I have some gaps in my knowledge. I left a comment on an example where I am unsure if this works.
https://docs.djangoproject.com/en/1.5/topics/python3/#unicode-literals

>Python 3 discourages using bytestrings, except for binary data or byte-oriented interfaces. Python 2 makes bytestrings and unicode strings effectively interchangeable, as long as they only contain ASCII data. Take advantage of this to use unicode strings wherever possible and avoid the b prefixes.