REF: require scalar in IntervalIndex.get_loc, get_value
cc @jschendel the casting of tuple->Interval on L819-820 seems sketchy, can you confirm if thats appropriate?

I'm also curious about the restriction on slice steps in _convert_slice_indexer, would be nice to have a comment there about why that is.

xref #31117.
thanks @jbrockmendel and @jschendel 