Ignore warning only on Python 3.4
This change to itersubclasses restores some determinism to it on Python 3.4.  See https://github.com/astropy/astropy/pull/1945#issuecomment-32522217

Conflicts:
	astropy/io/fits/util.py
Fix importing argparse on Python 2.6
Merge pull request #1973 from mdboom/misc/python3.4-compat

Omnibus Python 3.4 compatibility fixes