One question I would have about this is that `.parents` is a lazily-calculated sequence, not a list or a tuple, so it's not immediately obvious what the return type of a slice would be. I don't think it makes sense to return, e.g. a `_PathParents` object with fewer parts, but I don't know if there's any traditional choice here, other than that the result of slicing Sequence is another Sequence.

The PR returns a list, but I'm inclined to say we should return a tuple.