tox: Remove basepython

tox uses the python version where tox is installed by default, and we
no longer expect that the tests may run in env with python 2.

Change-Id: I242c957c1962d3565d8f85330abc1cc38bd3f95d

tox.ini

@@ -2,13 +2,8 @@
 minversion = 3.18.0
 envlist = py3,pep8
 skipsdist = false
-skip_missing_interpreters = true
-# this allows tox to infer the base python from the environment name
-# and override any basepython configured in this file
-ignore_basepython_conflict=true
 
 [testenv]
-basepython = python3
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -16,9 +11,6 @@ deps =
 commands =
   stestr run --slowest {posargs}
 
-[tox:jenkins]
-sitepackages = True
-
 [testenv:pep8]
 commands = flake8