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.)
As with Cartopy, this docstring would be even better as a comment.