Checklist

• I have verified that the issue exists against the master branch of Celery.
• This has already been asked to the discussion group first.
• I have read the relevant section in the
  contribution guide
  on reporting bugs.
• I have checked the issues list
  for similar or identical bug reports.
• I have checked the pull requests list
  for existing proposed fixes.
• I have checked the commit log
  to find out if the bug was already fixed in the master branch.
• I have included all related issues and possible duplicate issues
  in this issue (If there are none, check this box anyway).

Mandatory Debugging Information

• I have verified that the issue exists against the master branch of Celery.

Optional Debugging Information

isAlive was deprecated and removed in Python 3.9 . Celery has the deprecation warning that will become error in Python 3.9 .

https://travis-ci.org/celery/celery/jobs/628813003#L3262-L3263

Relevant CPython PR : python/cpython#15225