Add remaining Python3 __future__ imports

As a followup to #1365, this PR adds the `__future__` imports to all remaining files. Note, these files are not affected by these imports; all the affected files were included in the previous PR. Since it's a mass change of everything, I've put these in a separate PR.

Probably this will need to be rebased after #1365 is added, since I've noticed that some new files have been added. (None of them are affected by the imports, though.)