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: I58b115a17cf4293db6c8a0c1cd553f20fa4a3894
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,py27,py35,py36
+envlist = pep8,py27,py37
 skipsdist = True
 skip_missing_interpreters = True
 
@@ -23,13 +23,18 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = /bin/true
 
-[testenv:py36]
-basepython = python3
+[testenv:py35]
+basepython = python3.5
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
-[testenv:py35]
-basepython = python3
+[testenv:py36]
+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