Force python3 for zuul-tarball job
Force python3 for zuul-tarball job

Since both zuul and nodepool only support python3, use python3 to build
sdist and bdist_wheel.

Change-Id: I452574f9fd1c34aa60e81d63d2bc6771a30b51d6
Signed-off-by: Paul Belanger <e7139c8e6c4ea75cc1c287ce081b2dc0f3ab39ec@redhat.com>

openstack-infra/project-config,openstack-infra/project-config