docs: def _cmp_key(self, obj=None):
          """Comparison key for sorting results from all linters.
  
          The sort should group files and lines from different linters to make it
          easier for refactoring.
          """
          if not obj:
              obj = self
          line_nr = int(obj.line_nr) if obj.line_nr else 0
          col = int(obj.col) if obj.col else 0
          return (obj.path, line_nr, col, obj.msg)