docs: def parse_client_cert_pair(config_value):
      """Parses the client cert pair from config item.
  
      :param config_value: the string value of config item.
      :returns: tuple or none.
      """
      if not config_value:
          return
      client_cert = config_value.split(':')
      if len(client_cert) != 2:
          tips = ('client_cert should be formatted like '
                  '"/path/to/cert.pem:/path/to/key.pem"')
          raise ValueError('{0!r} is invalid.\n{1}'.format(config_value, tips))
      return tuple(client_cert)