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: I2ef60794a2606a7d7abfd413c6ba50689c52e67b
Story: #2005924
Task: #34236

.zuul.yaml

@@ -7,14 +7,14 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - release-notes-jobs-python3
     check:
       jobs:
         - openstack-tox-functional
         - openstack-tox-functional-py36
-        - placement-nova-tox-functional-py36
+        - openstack-tox-functional-py37
+        - placement-nova-tox-functional-py37
         - placement-nested-perfload:
             voting: false
         - placement-perfload:
@@ -42,15 +42,16 @@
       jobs:
         - openstack-tox-functional
         - openstack-tox-functional-py36
-        - placement-nova-tox-functional-py36
+        - openstack-tox-functional-py37
+        - placement-nova-tox-functional-py37
         - tempest-full-py3:
             irrelevant-files: *gate-irrelevant-files
         - grenade-py3:
             irrelevant-files: *gate-irrelevant-files
 
 - job:
-    name: placement-nova-tox-functional-py36
-    parent: nova-tox-functional-py36
+    name: placement-nova-tox-functional-py37
+    parent: nova-tox-functional-py37
     description: |
         Run the nova functional tests to confirm that we aren't breaking
         the PlacementFixture.

setup.cfg

@@ -17,6 +17,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [global]
 setup-hooks =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py{27,36},functional,functional-py36,pep8
+envlist = py27,py37,functional,functional-py37,pep8
 skipsdist = True
 # Automatic envs (pyXX) will use the python version appropriate to that
 # env and ignore basepython inherited from [testenv]. That's what we