Python 2 / 3 compatibility

This pull request consists of two commits, one for some automated PEP 8 fixes and the other with the actual fixes.  https://github.com/kevin-brown/async/commit/e015f93ccb7f4347404a435024abd82af8e19044 is the main commit to focus on.

In gitdb under Python 2, when using the latest async commit, this could not be imported at all.  This fixes the problems that came up, so now everything should work as it previously did.