Use the more modern six library to handle urllib, cStringIO, and cPickle.

The urlparse, urllib, and urllib2 modules have been significantly reworked in
Python 3 to provide a much more consistent interface.

Additionally, the 'fast' versions of StringIO and pickle are now merged with
the 'slow' versions, so in the future we'll be able to just use io.StringIO and
pickle without the fallbacks.

'six' has some adaptations to help with this for the time being. It's not as
pretty as I would like, but it works.

Testing done:
* Ran djblets unit tests.
* Ran Review Board unit tests.

Reviewed at https://reviews.reviewboard.org/r/4924/