More py3.x support

The Stein supported runtimes proposal [1] has chosen py36 over py35
due to the communities move to testing on Ubuntu 18.04, where py36
is the default runtime. We update tox.ini and setup.cfg accordingly.

We also add a py37 option to the tox.ini to facilitate future-looking
tests per the resolution on keeping up with Python 3 releases [2].

The py35 option will eventually need to be removed from tox.ini, but
this keeps it for now in order to allow time for folks to transition.

[1] https://review.openstack.org/611080
[2] https://review.openstack.org/613145

Change-Id: Id77fbd969bee5a6f3636be13362a2b89cb5888e8

setup.cfg

@@ -15,7 +15,7 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py35,py27,pep8
+envlist = py36,py27,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
@@ -41,6 +41,18 @@ commands =
   stestr run {posargs}
   stestr slowest
 
+[testenv:py36]
+commands =
+  {[testenv]commands}
+  stestr run {posargs}
+  stestr slowest
+
+[testenv:py37]
+commands =
+  {[testenv]commands}
+  stestr run {posargs}
+  stestr slowest
+
 [testenv:pep8]
 commands = flake8