Upgrade minversion to match python3

ignore_basepython_conflict flag is introduced in that version so we need
to raise it if using that flag.

Change-Id: Ie40535ebe42504aceb7c15efbce52038ce3f9b3b

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.1.1
 envlist = py36,py37,pep8
 skipsdist = True
 # Automatic envs (pyXX) will only use the python version appropriate to that
@@ -8,12 +8,12 @@ skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 whitelist_externals =
   rm
   make
 install_command = {[testenv:common-constraints]install_command}
-basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     OS_TEST_PATH=cyborg/tests/unit