Add requirement on packaging

[zonca]

Checking the version of photutils requires the packaging module.
See:

imexam/imexam/imexamine.py

Lines 62 to 68 in da7e3c5

	try:
	import photutils
	photutils_installed = True
	from photutils.centroids import centroid_com
	# account for API change
	from packaging import version
	photutils_version = version.parse(photutils.__version__)

I happened to have photutils installed, but it was not detected by imexam becase trying to import packaging was throwing ImportError.

[coveralls]

Coverage increased (+0.2%) to 38.334% when pulling 8d4da3e on zonca:patch-1 into d6f21d1 on spacetelescope:master.