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/339024bfb1d0a2b091e63d7a7ea23a1c63189f5c 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.