python3: update tox to check against both python versions (#40911)

Jenkinsfile

@@ -5,7 +5,7 @@ pipeline {
     stages {
         stage('Unit Tests') {
             steps {
-                sh 'tox -rv -e coverage'
+                sh 'tox -rv'
             }
             post {
                 always {

tox.ini

@@ -4,11 +4,10 @@
 # and then run "tox" from this directory.
 
 [tox]
-toxworkdir = {env:TMPDIR:/tmp}/tox-{env:USER}/zoo/
-envlist = py27
+toxworkdir = {env:TMPDIR:/tmp}/tox-{env:USER}/zoo/{env:BRANCH_NAME:}
+envlist = {py27,py3}-coverage
 
 [testenv]
-basepython = python2
 # django.contrib.auth is not tested it does not work with our templates
 whitelist_externals =
   /bin/mv