Allow the 'class' attribute to pass the sanitizer
Allow the 'class' attribute to pass the sanitizer

Fixes #3564

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