Chris Lamb, the current Debian Project Leader, has said that it will not be in stretch but could potentially be available in stretch-backports:

> If someone puts in the work, sure :) There isn't a "they" in Debian... it's, well, volunteers doing the actual grunt work if they feel it's needed..

Source: https://www.reddit.com/r/debian/comments/5a0gcf/will_python_...

With that said, I've personally packaged a simple backport of Python 3.6 (currently 3.6.2) for Debian stretch. There are pre-built Debian packages available in the releases tab, and a one-liner in the README for how to build them yourself: https://github.com/chriskuehl/python3.6-debian-stretch

You can install these alongside the regular python3.5 installation (it doesn't replace it).