install ipywidgets extension (conda does it but vanilla pip does not)

install.sh

@@ -162,6 +162,7 @@ else
 fi
 echo "installing python packages ($packages)…"
 python3 -m pip install -U $packages
+jupyter nbextension enable --py widgetsnbextension
 
 ### trust all our notebooks but make them read-only
 find . -name '*.ipynb' -print0 | xargs -0 jupyter trust