switch documentation job to new PTI
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: Id83de2e46044b501f99ae9da2c2ff7e5938431cd
Story: #2002586
Task: #24308

openstack/python-magnumclient