adds Python3 support
Remove options.attr API

This API was just fluff and didn’t serve a real purpose other than to
solidify the read-only intent of the already-normalized options.
Python3 had some trouble with this syntax, and it wasn’t even worth
debugging, so I removed it.
Remove debug line
Add Python 3 compatibility

Thanks are in order for the “six” library, @jacobvalenta, and Python 3
itself.
Add 0.7.0 changelog notes
Bump version number to 0.7.0