Remove io.fits dependence on 2to3

This gets all of the `astropy.io.fits` tests passing with the code written natively for Python 2 and 3, with some help from six. 

I don't want to merge this just yet as I think it will make doing other merges from PyFITS more difficult, but I just wanted see, as proof of concept, how much work it would take.  I will still merge this later, though it might require a rebase at that point.