Update code so it's python3 compatible
I wouldn't call postorius fully python3 compatible yet, because we don't have enough tests. But using this we at least prevent any python3 incompatible tests in the future.
This doesn't change any behavior of postorius, so it should be safe.
Changes include:
- Update strings
- Fix imports for modules that have moved in python3
- Update a couple of statements that were not compatible with python3