Fetcher using the wrong property in KafkaError

[joshhoegen]

2.1.0 is breaking since is_retriable doesn't exist: https://github.com/dpkp/kafka-python/blame/master/kafka/consumer/fetcher.py#L937

The property is retriable as seen in https://github.com/dpkp/kafka-python/blob/master/kafka/errors.py

2025-03-17T13:41:28.381-04:00 | File "/opt/bitnami/python/lib/python3.9/site-packages/kafka/consumer/group.py", line 1172, in __next__
-- | --
  | 2025-03-17T13:41:28.381-04:00 | return next(self._iterator)
  | 2025-03-17T13:41:28.381-04:00 | File "/opt/bitnami/python/lib/python3.9/site-packages/kafka/consumer/group.py", line 1144, in _message_generator_v2
  | 2025-03-17T13:41:28.382-04:00 | record_map = self.poll(timeout_ms=timeout_ms, update_offsets=False)
  | 2025-03-17T13:41:28.382-04:00 | File "/opt/bitnami/python/lib/python3.9/site-packages/kafka/consumer/group.py", line 679, in poll
  | 2025-03-17T13:41:28.382-04:00 | records = self._poll_once(inner_timeout_ms(), max_records, update_offsets=update_offsets)
  | 2025-03-17T13:41:28.382-04:00 | File "/opt/bitnami/python/lib/python3.9/site-packages/kafka/consumer/group.py", line 732, in _poll_once
  | 2025-03-17T13:41:28.382-04:00 | records, _ = self._fetcher.fetched_records(max_records, update_offsets=update_offsets)
  | 2025-03-17T13:41:28.382-04:00 | File "/opt/bitnami/python/lib/python3.9/site-packages/kafka/consumer/fetcher.py", line 353, in fetched_records
  | 2025-03-17T13:41:28.382-04:00 | self._next_partition_records = self._parse_fetched_data(completion)
  | 2025-03-17T13:41:28.382-04:00 | File "/opt/bitnami/python/lib/python3.9/site-packages/kafka/consumer/fetcher.py", line 937, in _parse_fetched_data
  | 2025-03-17T13:41:28.383-04:00 | elif error_type.is_retriable:
  | 2025-03-17T13:41:28.383-04:00 | AttributeError: type object 'UnknownError' has no attribute 'is_retriable'