code: def n_parent_paths(self, n):
          
          rv = os.path.relpath(os.path.join(self.path, '..'), os.path.join(self.path, *['..'] * (n+1)))
          return rv.replace('\\', '/')
docs: return parent paths of n levels.
          eg: File('a/b/c/d').n_parent_paths(2) == 'b/c'