Merge lp://staging/~cjwatson/storm/py3-dict into lp://staging/storm

Handle Python 3's changes to dict iteration methods.

In most cases I opted for a somewhat pedantic translation of d.iterfoo() to six.iterfoo(d) and d.foo() to list(six.iterfoo(d)), since there are enough cases where this matters either for performance or to avoid iterating over a dictionary while modifying it that it's simplest to be consistent.

There were a few cases where the Python 2 code was iterating over something like d.items() when it didn't need to do a copy; I left those cases alone, since they'll just become slightly more efficient under Python 3.