Test zuul with python3

Zuulv3 is python3 only, for now switch our tox environment to that.
Also force zuul_pip_executable to pip3 just to be safe.

Change-Id: I38403731936732bedd5772eb165f9d8bcd8dfa72
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

tests/test.yaml

@@ -15,6 +15,7 @@
 - hosts: test
   vars:
     rolename: "{{ lookup('pipe', 'pwd') | dirname | basename }}"
+    zuul_pip_executable: pip3
   pre_tasks:
     # Make sure OS does not have a stale package cache.
     - name: Update apt cache.

tox.ini

@@ -4,6 +4,7 @@ envlist = docs,linters
 skipsdist = True
 
 [testenv]
+basepython = python3
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt