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] celery/celery#6521

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

requirements.txt

@@ -4,7 +4,7 @@
 
 attrs
 attrs-strict
-celery >= 4.3
+celery >= 4.3, != 5.0.3
 Click
 elasticsearch > 5.4
 flask