Allow for "whitelisting" some packages as not requireing 2to3
Include six.py in extern
Add compatibility wrapper for urlopen functionality
Refactor io.votable to not require 2to3.
Fixes for Python 3.2
Merge pull request #1219 from mdboom/misc/avoid_2to3

Make astropy code compatible with Python 2 and 3 without requiring 2to3