Update Python 3 test runtimes for Train

One of train goals to have consistent python3 jobs.

https://governance.openstack.org/tc/goals/train/python3-updates.html

Change-Id: Iad68769d4ebed4a8e2dcb4eb0317bad40e6a27f2

setup.cfg

@@ -17,7 +17,8 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

zuul.d/project.yaml

@@ -4,8 +4,7 @@
       - 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: