Remove unused imports

I ran `flake` on the code and found many unused imports. This looks a little long because I used a single commit for every module just to make it easier to look through. You may perhaps prefer this squashed into something like standard modules in one commit, iris modules in another.

There are still a few unused imports that I left in, e.g., tests for matplotlib availability. I also left in some `iris` cross-references for backwards compatibility. But I did remove unused imports of the standard library; I assume no-one would use them in the odd manner of indirectly from `iris`.
Looks like this needs to go back in (probably due to #1281 which was merged recently).