Fixed tox warning.

I get:
  /home/florian/.local/venvs/tox/lib/python3.6/site-packages/tox/config.py:536: UserWarning: Conflicting basepython for environment 'py35'; resolve conflict or configure ignore_basepython_conflict
     testenv_config.envname, str(value), default
on newer tox versions.

Change-Id: I91ed0cf71ba81bf8bc4734ee310177aebbea32d7

.zuul.d/jobs.yaml

@@ -3,3 +3,9 @@
     parent: tox
     vars:
       tox_envlist: ansible-integration
+
+- job:
+    name: tox-py3
+    parent: tox
+    vars:
+      tox_envlist: py3

.zuul.d/project.yaml

@@ -7,9 +7,9 @@
       jobs:
         - ara-server-ansible-integration:
             voting: false
-        - tox-py35
+        - tox-py3
         - tox-pep8
     gate:
       jobs:
-        - tox-py35
+        - tox-py3
         - tox-pep8

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py35,pep8
+envlist = py3,pep8
 skipdist = True
 
 [testenv]
@@ -24,7 +24,7 @@ commands =
   flake8 ara hacking
   bandit -r ara --skip B303
 
-[testenv:py35]
+[testenv:py3]
 commands = python manage.py test ara
 
 [testenv:runserver]