docs: def validate_format_iso8601(validator, fieldname, value, format_option):
      """
      validates the iso8601 format
      raises value error if the value for fieldname does not match the format
      is iso8601 eg #"2007-06-20T12:34:40+03:00"
      """
      try:
          iso8601.parse_date(value)
      except ValueError:
          raise ValidationError(
              "Value %(value)r of field '%(fieldname)s' is not in "
              "'iso8601 YYYY-MM-DDThh:mm:ss(+/-)hh:mm' format" % locals())