Use "six" to abstract dictionary iteration and callable for python 2/3

I'm starting to work on porting our codebase to be compatible with both python
2 and 3. In python 3, items(), keys(), and values() are all iterators by
default and the old iter*() methods are gone. The "six" library includes
abstractions to handle these.

In addition, for some strange reason 'callable' is gone in python 3.0 and 3.1.
They brought it back in 3.2 (which may be a reasonable minimum), but for now,
to get the 2to3 tool to shut up, I'm using six's abstraction for that as well.

Testing done:
- Ran djblets unit tests
- Ran reviewboard unit tests

Reviewed at https://reviews.reviewboard.org/r/4893/