Support slice in interactive

[hoxbro]

Part 2 of the solution for #771.

It needs holoviz/holoviews#5352 to work.

The panel check is removed because panel is already imported at the top. Therefore, I think the check is unnecessary, but correct me if I'm wrong.

[maximlt]

test_interactive_slice is failing on Python 3.6. In these jobs the HoloViews version is 1.14.8 which doesn't yet have the fix to support slices that was added in holoviz/holoviews#5352 and made available in HoloViews 1.15.1. @hoxbro could you a guard checking HoloViews version after iff isinstance(op_arg, slice): and raising an error if it's too old, and skip that test for older HoloViews version?

[maximlt]

Thanks for making these last changes!