support py2 & py3

This makes the code execute properly under both Python 2 and Python 3.
We double up the unittests so that we don't actively break either.

We switch the unittests over to run under Python 3 by default too in
preparation for the future

Bug: 28298984
Test: unittests still pass
Change-Id: Ic67b441f98f902fe8ea3d276b0f4fca9c7cf9f25