Add Python 3 Train unit tests

This is a mechanically generated patch to ensure unit testing is in place
for all of the Tested Runtimes for Train.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Note: This diverges from the Train goal as it does not drop the
openstack-python35-jobs. As tempest is branchless it needs to continue
supporting Python 3.5.

Change-Id: Ie702ca637afb8229b6e605abcec25550bc9a0673
Story: #2005924
Task: #34240

.zuul.yaml

@@ -507,8 +507,7 @@
       - openstack-cover-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

setup.cfg

@@ -17,6 +17,8 @@ classifier =
     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 =