Refactor astropy.nddata to not need 2to3
Refactor astropy.stats to use six instead of 2to3
Refactor astropy.time to use six instead of 2to3
Prevent two lists from being created with Python 2
Removed debugging print statement
Removed more debug print statements
Merge pull request #1526 from astrofrog/nddata-stats-time-six

Refactor astropy.nddata, astropy.stats, and astropy.time to use six instead of 2to3