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

Depends-On:https://review.opendev.org/#/c/641878/
Change-Id: I41112851450e23a05cd7ad4ccdf43bc2ad678386

Author: pengyuesheng

.zuul.yaml

@@ -43,8 +43,7 @@
       - check-requirements
       - openstack-cover-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - openstackclient-plugin-jobs
       - publish-openstack-docs-pti
     check: