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: I7265f1293673af2ddd6f2a49ca521c663a4c6de5

tox.ini

@@ -1,20 +1,13 @@
 [tox]
 envlist = py3,pep8
-ignore_basepython_conflict = True
+minversion = 3.18.0
-minversion = 2.0
 
 [testenv]
-basepython = python3
 usedevelop = True
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
-commands =
-  stestr run --slowest {posargs}
-
-[tox:jenkins]
-sitepackages = True
 
 [testenv:pep8]
 commands = flake8