Update to python3

[dkirkby]

Known issues:

• [x] progressbar is not compatible with py3.x
• [x] from astropy.utils.compat import argparse no longer supported
• [x] use of iteritems()
• [x] make html in docs is failing
• [x] notebooks in examples/nb use print without ()
• [x] various basestring / bytes issues.

[dkirkby]

Use progressbar2 as a drop-in py2/3 compatible replacement for progressbar.

[dkirkby]

Replace X.iteritems() with:

from six import iteritems
...
iteritems(X)