Log Message:
py-celery: updated to 4.2.1

4.2.1:
- **Result Backend**: Fix deserialization of exceptions that are present in the \ 
producer codebase but not in the consumer codebase.
- **Message Protocol Compatibility**: Fix error caused by an invalid (None) \ 
timelimit value in the message headers when migrating messages from 3.x to 4.x.
- **Result Backend**: Fix serialization of exception arguments when exception \ 
arguments are not JSON serializable by default.
- **Worker**: Fixed multiple issues with rate limited tasks
  Maintain scheduling order.
  Fix possible scheduling of a :class:celery.worker.request.Request with the \ 
wrong :class:kombu.utils.limits.TokenBucket which could cause tasks' rate limit \ 
to behave incorrectly.
  Fix possible duplicated execution of tasks that were rate limited or if \ 
ETA/Countdown was provided for them.
- **Worker**: Defensively handle invalid timelimit header values in requests.