Just copying or symlinking celery-3.1.19-test.patch to celery-3.1.20-test.patch (and rebuilding the Manifest) does the job.
commit 9792fe63a045e96682cf47b346b91ac2ac4fc4a4 Author: Justin Lecher <jlec@gentoo.org> Date: Thu Jan 28 09:48:57 2016 +0100 dev-python/celery: Add missing patch Gentoo-Bug: https://bugs.gentoo.org/show_bug.cgi?id=573104 Package-Manager: portage-2.2.27 Signed-off-by: Justin Lecher <jlec@gentoo.org> https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=9792fe63a045e96682cf47b346b91ac2ac4fc4a4