Add py36 testenv

Python 3.6 is installed by default in Ubuntu 18.04 LTS.
According to Python2 Deprecation Timeline [1] and Python3-first Goal [2],
we should have py36 testenv ready for it.

This change adds a non-voting job for checking the 3.6
upper-constraints, and a voting job for py36.

Once we're confident we have all the bits in place for 36-uc checking we
can promote that job to voting.

[1] https://governance.openstack.org/tc/resolutions/20180529-python2-deprecation-timeline.html
[2] https://review.openstack.org/#/c/575933

Change-Id: Ia06c1ab217e80902adb607a3bc6063aae66f48bf

.zuul.d/jobs.yaml

@@ -34,6 +34,16 @@
     vars:
       tox_envlist: py35-check-uc
 
+- job:
+    name: requirements-tox-py36-check-uc
+    parent: openstack-tox-py36
+    description: |
+      Run test for requirements project.
+
+      Uses tox with the ``py36-check-uc`` environment.
+    files: ^upper-constraints.*txt$
+    vars:
+      tox_envlist: py36-check-uc
 - job:
     name: requirements-tox-validate-projects
     parent: openstack-tox

.zuul.d/project.yaml

@@ -19,6 +19,8 @@
             voting: false
         - requirements-tox-py27-check-uc
         - requirements-tox-py35-check-uc
+        - requirements-tox-py36-check-uc:
+            voting: false
         - requirements-tox-validate-projects
         - cross-cinder-py27
         - cross-cinder-py35
@@ -36,6 +38,7 @@
         - cross-osc-build-sphinx-docs
         - cross-swift-py27
         - cross-swift-py35
+        - openstack-tox-py36
     gate:
       queue: integrated
       jobs:
@@ -73,3 +76,4 @@
         - cross-osc-build-sphinx-docs
         - cross-swift-py27
         - cross-swift-py35
+        - openstack-tox-py36

tox.ini

@@ -25,6 +25,12 @@ install_command = pip install -U {opts} -c {toxinidir}/upper-constraints.txt {pa
 deps = -r{toxinidir}/upper-constraints.txt
 commands = check-conflicts {toxinidir}/upper-constraints.txt {toxinidir}/upper-constraints-xfails.txt
 
+[testenv:py36-check-uc]
+basepython = python3.6
+install_command = pip install -U {opts} -c {toxinidir}/upper-constraints.txt {packages}
+deps = -r{toxinidir}/upper-constraints.txt
+commands = check-conflicts {toxinidir}/upper-constraints.txt {toxinidir}/upper-constraints-xfails.txt
+
 [testenv:venv]
 commands = {posargs}
 
@@ -86,6 +92,9 @@ basepython = python3.4
 [testenv:py35]
 basepython = python3.5
 
+[testenv:py36]
+basepython = python3.6
+
 [testenv:babel]
 # Use the local upper-constraints.txt file
 deps = Babel