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: Ic50d2199a606282bcf6c685e19ea025d99ca5dc6

tox.ini

@@ -1,12 +1,9 @@
 [tox]
 envlist = py3,pep8
-ignore_basepython_conflict = True
+minversion = 3.18.0
-minversion = 2.0
 
 [testenv]
-basepython = python3
 usedevelop = True
-install_command = pip install {opts} {packages}
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -14,9 +11,6 @@ deps =
 commands =
   stestr run --slowest {posargs}
 
-[tox:jenkins]
-sitepackages = True
-
 [testenv:pep8]
 commands = flake8