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.
I find it a bit weird that the end is just a space, considering the colons between key and value, but maybe neither key nor value contain spaces, and it's clear enough?