Change astropy version requirement statement: latest stable or dev
Add astroquery.extern.six for easy Python 3 compatibility

Should be removed once astropy 0.2 support is dropped,
because astropy 0.3 contains astropy.extern.six which we can use.
Use astroquery.extern.six instead of astropy.extern.six
Merge remote-tracking branch 'cdeil/p3' into py3compat