Python3 "safe" fixes.

I ran [futurize](http://python-future.org/) on iris, applying _only_ the ["safe" fixes](http://python-future.org/futurize.html#stage-1-safe-fixes). In fact, this PR is even more conservative since I also left out any changes that would add `__future__` imports, as well as extraneous whitespace changes. I figure doing it this way will make it easier to review, since these are mostly small commits.