astropy.utils.compat.argparse not working for Python 3.1

@mdboom - I noticed you made some fixes recently to ``astropy.utils.compat.argparse`` but there is one case where it still doesn't work, which is for Python 3.1 (since ``argparse`` also doesn't exist there). However, ``argparse_py2`` doesn't get installed with Python 3, so what do you think is the best way to work around this? Should we add a new ``argparse_py3`` which is the back-ported Python 3.2 ``argparse``?