While we're generally pretty good about using six.iter* functions, we

had a few places where we weren't using them, and instead were calling

methods like .values() and .items() directly, producing different

results on Python 2.x and 3.x. This change makes use of six in these

locations, and documents one location where it appears we can use it but

actually can't due to the object type.

Unit tests pass (along with other changes) on Python 2.7, 3.4, 3.5, and 3.6.