Avoid circulare dependency between groups and users

Extract the _PREFIX constants to their own file and only import
groups within the method that uses it. While the issue doesn't show
up under the current setup and with Python 2, when fixing the imports
for Python 3 it does appear.

Change-Id: I419bae3d0a088fce5e78c5890aab66423f280465
Various fixes for Python 3

This applies the output of the modernisation tool to the repo, plus
some further manual fixes. It fixes iteration, type checking and
import issues so the source is valid for both Python 2 and 3.

Change-Id: Ia98dae5f5c3e3066412e052b6279575549643120
Cope with the configparser module being lowercase on Python 3
Move delayed imports so they work

Change-Id: Ia5bb0ca8192a170edb8465c8276ff0cada4e3aa5
Add a readme which details the dependencies