Use importlib.machinery.all_suffixes if possible for open files test
Test for open files with travis
Support Python 2.6's lack of importlib

This change is made on a distro without python2.6 support, so good luck
Import importlib.machinery in open-files pytest plugin
Check for open files as part of all the base tests
Merge pull request #3096 from sargas/fix-python33-open-files

Fix --open-files test with Python 3.3 and newer
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.
Applied review comments, updated tests and docs. Added functions to calculate Wien's displacement law and blackbody sampleset.
Changed xfail to skipif for scipy test. Fixed broken doc links.
Renamed functions as suggested by Erik T.