HTML sanitization removes class attribute in descriptions

[charwick]

As of Ipywidgets 8.0, Ipywidgets removes the class attribute from HTML in the description attribute (#2785). This seems like important functionality.

i = interactive(func, val=False)
circle='<span class="projectname_circle" style="background:#d73229"></span>' if circle is not None else ''
i.children[0].description_allow_html = True
i.children[0].description = circle+'Title'

The resulting markup:

<label class="widget-label-basic">
    <input type="checkbox">
    <span title="<span class=&quot;projectname_circle&quot; style=&quot;background:#d73229&quot;></span>Jam">
        <span style="background:#d73229"></span>Title
    </span>
</label>

[jasongrout]

I've submitted a PR adding class back in to the allow list: #3565