Introduce six to address python 3 compatibility.

This is a pretty blind python 3 compatibility conversion that
may well have completely missed the point of any or all of the
code it touches. It does, however get all but one of the tests
passing on python 3.3 (with django 1.6 pre-alpha).

I do not understand why `TestLayoutObjects.test_tab_helper_reuse`
does not pass.