[python] Make `Slice` protocol not `runtime_checkable` #128

Since `range` has the members `start`/`stop`/`step`, that meant that `isinstance(range(x), Slice)` would return True, when a `range` is *not* a `Slice`. Instead, we only provide the `is_slice_of` function (originally from `tiledbsoma`) to perform the appropriate type check. This also unrestricts `Slice`s from being `Comparable`, since the built-in `slice` type does not have this restriction.