Whether we use six or not, I think we should add utils.compat, though I would call it utils.python and we can put all kinds of small python helper functions in there. It would be nice to at least have `is_string` that does the above, and `is_number` that checks if something is an int, float, or long (and maybe complex?). However, both of these are in a sense compatibility related, since long becomes int in Python3. So I'd be fine with utils.compat as the name, if people don't like utils.python or find it too vague. Thoughts?