Update Python 3 test runtimes for Train

Following the process set down in the TC resolution on updating Python
versions:

https://governance.openstack.org/tc/resolutions/20181024-python-update-process.html

Change-Id: If36099eb2ab0c34c18544a0996d2bbaea255b539

zuul.d/project.yaml

@@ -6,8 +6,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check: