code: def _deprecation_notice(cls):
          
          _deprecation_msg = (
              '{name} {type} is deprecated. '
              'It will be removed in the next version. '
              'Use saml2.cryptography.symmetric instead.'
          ).format(name=cls.__name__, type=type(cls).__name__)
          _warnings.warn(_deprecation_msg, DeprecationWarning)
docs: Warn about deprecation of this class.