Common Python 2 / 3 code base without 2to3

This PR sets `use_2to3=False` in `setup.py` and fixes the few cases where the codebase wasn't simultaneously Python 2 / 3 compatible already.
@astrofrog This was causing a test failure on Python 3.2:
https://travis-ci.org/aplpy/aplpy/jobs/30545733#L396

Do you think simply dropping the `u` prefix here is OK? It seems to work on Python 2 and 3.