Remove dependency on six and enable sphinx tests on Python 3

I'd be okay with not bothering with the `iteritems` helper at all and just calling `.items()` in all these cases (unless there are any cases you can find where it's absolutely necessary to return an iterator).  Most of the dictionaries we're talking about here are smallish, and not being used in any performance-critical context, so I don't see that it's a big deal to use `.items()` (on Python 3 that will still return an iterator; on Python 2 who cares?)