docs: def n_relative_paths(self, n):
          """
          return relative paths of n levels, including basename.
          eg: File('a/b/c/d').n_parent_paths(3) == 'b/c/d'
          """
          rv = os.path.join(self.n_parent_paths(n-1), self.basename)
          return rv.replace('\\', '/')