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 openstack-python35-jobs is added since this charm is supported
on Xenial.

Change-Id: Ie5a0ac7be4ecb95c109564368710c892f085a844
Story: #2005924
Task: #34228

.zuul.yaml

@@ -1,5 +1,6 @@
 - project:
     templates:
       - python-charm-jobs
-      - openstack-python36-jobs
+      - openstack-python35-jobs
+      - openstack-python3-train-jobs
       - openstack-cover-jobs

tox.ini

@@ -3,7 +3,7 @@
 # within individual charm repos.
 [tox]
 skipsdist = True
-envlist = pep8,py36
+envlist = pep8,py37
 skip_missing_interpreters = True
 
 [testenv]
@@ -33,11 +33,21 @@ deps =
 whitelist_externals = true
 commands = true
 
+[testenv:py35]
+basepython = python3.5
+deps = -r{toxinidir}/test-requirements.txt
+commands = stestr run {posargs}
+
 [testenv:py36]
 basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
+[testenv:py37]
+basepython = python3.7
+deps = -r{toxinidir}/test-requirements.txt
+commands = stestr run {posargs}
+
 [testenv:pep8]
 basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt