This allows us to handle Review Board development on Python 3.x without

worrying about end users installing it, or having trouble with

dependencies.
We can now specify dependencies based on Python version ranges in

reviewboard/dependencies.py. Right now, we choose different versions

of Django, Haystack, and Django-Evolution, but we'll likely need to bump

other dependencies as well.
dev-requirements.txt no longer tries to install Bazaar or Mercurial on

Python 3.x, due to incompatibility here.

Set up development environments on 2.7, 3.5, 3.6, and 3.7. Verified the

installed package versions.