Don't run 2to3 for Python 3

As mentioned in https://github.com/astropy/package-template/pull/93 it would be nice if Astropy and all affiliated packages worked with a single Python 2 / 3 codebase.

This is usually not hard to achieve ... set `use_2to3=False` in `setup.py` and then once run `2to3` on your package and manually go through the suggested changes to see which ones make sense.