requirement: Adapt celery requirements

This adapts the celery requirements to the last known where our builds are fine. Currently, 5.0.3 got released and this ends up making all the swh modules relying on tasks timeout. A bug upstream is opened [1].

In the mean time, this workaround fixes [2] and most probably the remaining swh builds.

• [1] https://github.com/celery/celery/issues/6521

• [2] https://jenkins.softwareheritage.org/job/DSCH/job/tests/1132/console

Test Plan

tox

Another test on current failing build (swh-indexer):

(swh-indexer) $ .tox/py3/bin/pip install ../swh-scheduler
(swh-indexer) $ tox -- -x -s
...
330 passed, 15 skipped, 1 warning in 56.79s

-- summary --

  black: commands succeeded
  flake8: commands succeeded
  mypy: commands succeeded
  py3: commands succeeded
  congratulations :)

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Migrated from D4672 (view on Phabricator)