Make astropy.modeling compatible with Python 2 and 3 using six
Added __future__ imports
Use the range function from six
Fix metaclass use
Removed duplicate functions (_model_to_fit_params is defined again lower down, and _fitter_to_model_params is defined in fitting.py)
Fixed formatting of parameter names now that we are no longer using 2to3
Fixed documentation tests
Merge pull request #2035 from astrofrog/modeling-six

Make astropy.modeling compatible with Python 2 and 3 using six