docs: def _list_class_vars(cls, exclude=None):
      """
      Return a dict of all 'regular' (i.e. not prefixed ``_``) class attributes.
      """
      cvars = {k: v for k, v in vars(cls).items()
               if not k.startswith('_')}
      cvars = deepcopy(cvars)
      if exclude is not None:
          for e in exclude:
              cvars.pop(e)
      return cvars