Compatibility changes with bokeh 3.0

[hoxbro]

For now, this is only done to be able to run panel tests in https://github.com/holoviz/panel/pull/3752.

With these changes, it is also possible to run the following with Bokeh version 3.0:

import holoviews as hv
hv.extension("bokeh")

I have been pretty brutal and removed what is not needed, it could be added back in the future.

Widgetbox does not exist anymore, see https://github.com/bokeh/bokeh/issues/9334.