ipywidgets: support custom serializers, so now datepicker works. (see https://github.com/sagemathinc/cocalc/issues/3789)
ipywidgets: support custom serializers, so now datepicker works. (see https://github.com/sagemathinc/cocalc/issues/3789)

sagemathinc/smc,sagemathinc/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc