python 2 to 3: https://portingguide.readthedocs.io/en/latest/numbers.html#division

python 2 to 3: https://portingguide.readthedocs.io/en/latest/iterators.html#new-behavior-of-range

Where there's a chance of performance issues with switching from the python 2 xrange to python 2 range, I've imported `from six.moves import range` to preserve the iterator behavior.
However in places where there's not a performance issue, I've left out the six import so that there's less code churn after switching to python 3.

python 2 to 3: https://portingguide.readthedocs.io/en/latest/builtins.html#the-print-function