Remove __future__ imports

[ketch]

Since we no longer support Python 2, we can remove these, which are still present in a very large number of files.

This should be done for the whole PyClaw codebase in one PR that can be squashed to a single commit.

Thanks to @pavelkomarov for suggesting this with #726.

[pavelkomarov]

subl `grep -IR . -e "__future__" | cut -d: -f1` opened them all, and I chopped the future out.

[pavelkomarov]

We can close this now.