get-pip.py is fixed for py2.7 in
https://review.opendev.org/c/openstack/devstack/+/772462
This commit support it for py3.5 as devstack stable/train and stein
still support ubuntu xenial which has py3.5
Also using Temepst 23.0.0 for xenial platform job which is
compatible version on py3.5.
Closes-Bug: #1913283
Change-Id: Icd40e41b5f3ef9a621ca1b9739875fcdd31abd74