Add a step to make sure the widgetsnbextension package is uploaded.
Add a step to make sure the widgetsnbextension package is uploaded.

Fixes #2137

jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets