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

Change-Id: I0fb852f85e435c1ec53b4e250255534c708eab0b
Story: #2005924
Task: #34242

.zuul.yaml

@@ -9,8 +9,7 @@
 
 - project:
     templates:
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - build-openstack-docs-pti
     check:
       jobs:
@@ -19,11 +18,11 @@
             irrelevant-files:
               - ^doc/.*$
         - releases-tox-list-changes
-        - openstack-tox-py35:
+        - openstack-tox-py36:
             irrelevant-files:
               - ^deliverables/.*$
               - ^doc/.*$
-        - openstack-tox-py36:
+        - openstack-tox-py37:
             irrelevant-files:
               - ^deliverables/.*$
               - ^doc/.*$
@@ -37,11 +36,11 @@
             timeout: 3600
             irrelevant-files:
               - ^doc/.*$
-        - openstack-tox-py35:
+        - openstack-tox-py36:
             irrelevant-files:
               - ^deliverables/.*$
               - ^doc/.*$
-        - openstack-tox-py36:
+        - openstack-tox-py37:
             irrelevant-files:
               - ^deliverables/.*$
               - ^doc/.*$

setup.cfg

@@ -14,7 +14,8 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages = openstack_releases

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.0
-envlist = py35,validate,pep8,bashate,docs
+envlist = py37,validate,pep8,bashate,docs
 skipdist = True
 ignore_basepython_conflict=true