Date parameter cannot be edited with ipywidgets<6

[ceball]

ipywidgets introduced DatePicker in version 6.0. When using paramnb with ipywidgets before 6.0, you get a text widget - and when the string is passed to param.Date you get a validation error because it's not a datetime.

I guess the two options are either to ignore this (things change so fast we can't support old ipywidgets, right?) or for paramnb to create a datetime from the string.

[philippjfr]

Having tried @jbednar's recent notebook I've also noticed that it's not working correctly even with ipywidgets 6.0 at least under some conditions. Haven't yet investigated further.