Drop widgetsnbextension as a dependency.

Author: Alan

python/ipywidgets/setup.cfg

@@ -37,7 +37,6 @@ install_requires =
   comm>=0.1.3
   ipython>=6.1.0
   traitlets>=4.3.1
-  widgetsnbextension~=4.0.14
   jupyterlab_widgets~=3.0.15
 
 [options.extras_require]