import reduce function for jupyter use

Hi Sam,

I'm a big fan of your pytide package. I used this package a lot in the old version of python (2.7).  However, in a jupyter version (python 3.0) a issue emerged asking for properly import reduce function. 
This issue is resolve by inserting this line in constituent.py:
`from functools import reduce`

Maybe you can upgrade the code with this line ;)
Kind regards
Mabel

