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 that python35-charm-jobs is retained since this charm is supported
on Xenial.

Change-Id: I8260eeaccd4539ac81fa4dc1805262d1d083e1c7
Story: #2005924
Task: #34228

.zuul.yaml

@@ -1,4 +1,5 @@
 - project:
     templates:
       - python35-charm-jobs
+      - openstack-python3-train-jobs
       - openstack-cover-jobs

tox.ini

@@ -2,7 +2,7 @@
 # This file is managed centrally by release-tools and should not be modified
 # within individual charm repos.
 [tox]
-envlist = pep8,py3{5,6}
+envlist = pep8,py37
 skipsdist = True
 
 [testenv]
@@ -33,6 +33,11 @@ basepython = python3.6
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
+[testenv:py37]
+basepython = python3.7
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+
 [testenv:pep8]
 basepython = python3
 deps = -r{toxinidir}/requirements.txt