pin billiard dependency to versions < 4

Billiard version 4 is known to cause trouble in the git-loader's test
suite because of specific requirements set by the celery package:

pkg_resources.ContextualVersionConflict: (billiard 4.0.2
 (/home/stsp/.virtualenvs/swh/lib/python3.9/site-packages),
Requirement.parse('billiard<4.0,>=3.6.4.0'), {'celery'})

Until that is fixed, pin billiard to known-working versions.