No longer use 2to3 when building astropy. Fixes #2033.
Merge pull request #2033 from astrofrog/no-2to3

Make all astropy code compatible with Python 2 and 3 without requiring 2to3
added planck function to functional_models
update to planck function
added planck and test_planck() functions
back to baseline prior to moving planck stuff to new subpackage
Added new subpackage. Added Planck function by @skendrew from #1480 and related comments in that PR.
Modified planck function based on the one in pysynphot. Added tests and documentation.