Remove six dependency

So, six turned out to be a bad choice. It was originally brought in to
add support for python 3. That transition was abandoned, as the
differences between python2 and python3 unicode handling are too
different to do this simply. However, I thought leaving six in would be
harmless, as eventually I'd like to support python3, if the demand
presents itself.

This turned out to be a bad idea because six has at least two nasty
dependencies - TKinter and winreg/_winreg. The former was fixed by
installing the distrubtion package python-tk. This was frustrating,
because when I tested six on machines that *happened* to have python-tk,
I didn't realize this dependency existed. I gave up after trying to
solve the winreg dependency. It's bad enough to install a gui library
for a webapp running on a server, but an OS dependency (winreg adds
MS windows registry manipulation to python) is even worse.

Maybe I'm doing something wrong here, but for now, six has to go.