Use floor division for index calculation

The `collection[index]` statement at line 170 of util.py fails with `TypeError: list indices must be integers, not float` in Python 3 implementations as a result of the index calculation using the true-division operation, which returns a float regardless of its operand types. Presumably this is a result of the change in `/` operator semantics between Python 2 and Python 3.

Explicitly using the floor-division operator will solve this issue for Python 3 while leaving Python 2 implementations unaffected.