Add Python 2.6 support back in (?)

Hi, thanks for etcaetera!

I'm hoping to use it in a Python 2.6->3.4 project, and needed to fix the below handful of 2.7-isms for it to install and import under 2.6.

Given your git history it looks like 2.6 support was (partially - it's still in `CONTRIBUTING.rst`) dropped when you added 3.x support. However, as the diff shows, the only nontrivial change needed for 2.6 to work is rolling back the dict comprehensions. I'm hoping you find it a worthwhile tradeoff (I researched for my own projects & found 2.6 still has a nontrivial install base.)

I'm able to install, import `config.Config`, and run `runtests.py`, all successfully on Python 2.6, 2.7 and 3.3.