Use openstack-python3-train-jobs for python3 test runtime
Use openstack-python3-train-jobs for python3 test runtime

Depends-On:https://review.opendev.org/#/c/641878/

Change-Id: I4fe08fcd45e5f4ea3431c9cd4072c78d9d95a7b2

openstack/mistral-dashboard,openstack/mistral-dashboard,openstack/mistral-dashboard