Add missing zuul trigger to pypi upload job.

Change-Id: I1d3565c8b26bf03e7746a8b86728158ef532b0b8
Reviewed-on: https://review.openstack.org/19652
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins