Use lib2to3's fix_dict to help with list(dict.items())

Also helps cover dict.viewitems() and friends.

Closes #35
This isn't strictly correct, because there are things you can do with a view that you can't do with an iterator - iterate over it multiple times, membership tests without exhausting it, set operations. However, I've never seen Python 2 code using these features - I didn't even know of the `.viewitems()` etc. methods until now. So I'm happy with this as a compromise, until six supports creating views.