[Rivet] python2 change

[David Grellscheid]

> Does anybody have experience with the __future__ compatibility layer?

They change the behaviour of features, so that a python2 interpreter
does the right thing with python3 code:

As the first lines in the file write:

from __future__ import print_function # works from 2.6
from __future__ import division # from 2.5
from __future__ import with_statement # from 2.5
from __future__ import # ... whatever else you need, see ...

# https://docs.python.org/2/library/__future__.html

  David