Pinned Werkzeug version to one that doesn't break flask-session.
I created the issue on flask-session's project https://github.com/fengsp/flask-session/issues/112. As of now, I haven't received a response. I also e-mailed the maintainer, but to no avail.
In worst case we need to fork this. Latest commit is from Feb 9, 2017, so doesn't give much hope that somebody will address this issue. :(