Run all jobs by default using python3

This patch implements the community wide goal to run all jobs by
default using python3.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: Ie819fff9c792ea1b82262a8ec67caccf24592f8d

.zuul.yaml

@@ -2,7 +2,10 @@
     templates:
       - check-requirements
       - openstack-python-jobs
-      - openstack-python35-jobs-nonvoting
+      - openstack-python35-jobs
+      - openstack-python36-jobs
+      - publish-openstack-docs-pti
+      - release-notes-jobs-python3
     check:
       jobs:
         - openstack-tox-lower-constraints

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py35,py27,pypy,pep8
+envlist = pep8,py27,py35,py36
 skipsdist = True
 
 [testenv]
@@ -33,8 +33,14 @@ commands =
     {[testenv]commands}
     ostestr '{posargs}'
 
+[testenv:py36]
+basepython = python3.6
+commands =
+    {[testenv]commands}
+    ostestr '{posargs}'
+
 [testenv:pep8]
-#basepython = python3
+basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]