ENH: Always do true division on Python 2.X

Close #5356.

Note: this version actually forces `a / b` to always do truediv as well. It's
less complicated to maintain if we do it that way, but it's also possible to
have this `a / b` use div and `a.div(b)` use truediv, just might be strange.
This is definitely different from numpy behavior.

We also may need to add the division future import to more places.