slack integration: wait and retry on rate limited error

[ungarj]

When many jobs run concurrently, we occasionally hit the rate limit by Slack:

ERROR mapchete_hub.observers.slack_messenger The request to the Slack API failed. (url: https://slack.com/api/chat.postMessage)
The server responded with: {'ok': False, 'error': 'ratelimited'}
Traceback (most recent call last):
  File "/usr/local/src/mapchete_hub/mapchete_hub/observers/slack_messenger.py", line 160, in _send
    response = self.client.chat_postMessage(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/slack_sdk/web/client.py", line 2710, in chat_postMessage
    return self.api_call("chat.postMessage", json=kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/slack_sdk/web/base_client.py", line 157, in api_call
    return self._sync_send(api_url=api_url, req_args=req_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/slack_sdk/web/base_client.py", line 188, in _sync_send
    return self._urllib_api_call(
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/slack_sdk/web/base_client.py", line 319, in _urllib_api_call
    ).validate()
      ^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/slack_sdk/web/slack_response.py", line 199, in validate
    raise e.SlackApiError(message=msg, response=self)
slack_sdk.errors.SlackApiError: The request to the Slack API failed. (url: https://slack.com/api/chat.postMessage)
The server responded with: {'ok': False, 'error': 'ratelimited'}