Fix gate interop-tempest-consistency job

interop-tempest-consistency job still use the python2.7
env to run as default basepython in testenv is py2.7.

Requirement has dropped the py2.7 caps now
- I2d58d1b524764ea1d46d6ab3a382783f6a002b34

and interop-tempest-consistency job started failing on interop
and Tempest gate. To unblock gate on both project we need to move
the default tox env of interop to python3.

Change-Id: I40770cf254369808c2e420dae6c2ec52bf47b777

tox.ini

@@ -1,17 +1,17 @@
 [tox]
 envlist = docs,doc8,pep8,jsonlint,jsonschema,consistency
-minversion = 1.6
+minversion = 3.1
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
-basepython=python2.7
+basepython=python3
 setenv =
    VIRTUAL_ENV={envdir}
 deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/test-requirements.txt
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:doc8]
@@ -19,12 +19,10 @@ commands =
   doc8 -e .rst doc/source
 
 [testenv:docs]
-basepython = python3
 commands=
     sphinx-build doc/source doc/build/html
 
 [testenv:pep8]
-basepython = python3
 ignore = W504
 deps =
   flake8