Add (some) Python3 __future__ imports

Added both `print_function`, and `absolute_import` from `__future__`. Note, the changes are _only_ to the files where these imports would make a difference. I did not change every file, because I figured that would bloat the PR substantially.
It's one of the idioms that 2to3 converts, but I don't think it would have broken anything in Python 3.