Explicitly import reduce from functools

[saatviku]

This minor change is intended to make library run error free in Python 3.

Python 3 dropped support for reduce() without using functools.

Reduce() is available in functools in Python2 as well.
https://docs.python.org/2/library/functools.html#functools.reduce

"This is the same function as reduce(). It is made available in this module to allow writing code more forward-compatible with Python 3."

Lack of this will throw an error in Python 3 : NameError: name 'reduce' is not defined

[parsifal-47]

Thanks for contributing!
Hopefully someone with write access would merge it