Refactor: Allow only pathlib.Path as type for the path field in ListEntrymodel

[dl6nm]

I'll suggest to use a pathlib.Path type only instead of also allowing a str to set in ListEntry model. This would be more straight forward for returning only one type, instead of guessing the right one.

In case of getting relative_paths out of the model, we can then use one of the following implementations in _PathTreeGenerator:

• p.relative_to(self._root_dir), if relative_paths=True
• p.relative_to('.'), if relative_paths=False

This can makt it easier to determine the absolute or relative path of a ListEntry in PathTree(...).tree().