Use a modern PBR package
Use a modern PBR package

The 2.0.0 is breaking in that it removes the use of warnerrors in
build_sphinux.

requestsexceptions isn't using that feature, so it shoudln't break.

The cap on pbr is preventing other OpenStack projects that would like to
use pbr 2.0.0 (and sphinx 1.5.1) from doing so as it breaks
co-installability with requestsexceptions

Change-Id: Id7ed73ec18580a9649f67b42f369c8820573b4a5
Related-Bug: 1668848

openstack-infra/requestsexceptions