Bump celery from 4.3.0 to 4.4.0

Bumps [celery](https://github.com/celery/celery) from 4.3.0 to 4.4.0.
- [Release notes](https://github.com/celery/celery/releases)
- [Changelog](https://github.com/celery/celery/blob/master/Changelog.rst)
- [Commits](https://github.com/celery/celery/compare/v4.3.0...4.4.0

)

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