No 2to3 for Python 3 builds

@astrofrog Is simply setting `use_2to3` in `setup.py` the right way to not run 2to3 for an Astropy affiliated package?
It seems to work but this simple method is currently not mentioned in the [astropy docs](http://docs.astropy.org/en/latest/development/codeguide.html#writing-portable-code-for-python-2-and-3).