switch documentation job to new PTI

This is a mechanically generated patch to switch the documentation
jobs to use the new PTI versions of the jobs as part of the
python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: I03acea5710dc85938d1b77bdc81c3f8704f64659
Story: #2002586
Task: #24345

.zuul.yaml

@@ -3,7 +3,7 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - check-requirements
-      - release-notes-jobs
+      - release-notes-jobs-python3
     check:
       jobs:
         - openstack-tox-lower-constraints