ipywidgets 8.x conflicts

[mkeilman]

Discovered with a fresh local install - viewer code code breaks with ipywidgets 8, at least here:

~/src/radiasoft/jupyter_rs_radia/jupyter_rs_radia/radia_viewer.py in __init__(self, mgr)
    212         self.model_data = {}
    213         self.mgr = radia_tk.RadiaGeomMgr() if mgr is None else mgr
--> 214         self.on_displayed(self._radia_displayed)
    215         self.vtk_viewer = vtk_viewer.Viewer()
    216 

AttributeError: 'RadiaViewer' object has no attribute 'on_displayed'

on_displayed was removed from the base widget class. We currently have ipywidgets 7.6.5 installed, so we're OK for now, but we'll have to deal with this before moving to 8.