I'm writing this down because two of us had the same problem.
I installed vanilla Python 3.6, then pip install jupyter[notebook] matplotlib
In order to get the javascript for ipywidgets to work, I had to run jupyter nbextension enable --py --sys-prefix widgetsnbextension, and restart jupyter (I think I had to do this a couple of times before it worked)
I'm writing this down because two of us had the same problem.
pip install jupyter[notebook] matplotlibjupyter nbextension enable --py --sys-prefix widgetsnbextension, and restart jupyter (I think I had to do this a couple of times before it worked)