keep previous version (unless on master) and update to python 3

Hi ralfth!

Stepanv and I did a couple of changes you might be interested in.
The biggest one is a change in philosophy, keeping our previous version in the current branch, unless on master. This allows us to use git flow (release/hotfix)-finish better.
The change to python 3 might be considered a big one too if you're on v2...