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: I19a02201fa99efd520e808b02e95306f72a999e0

tox.ini

@@ -1,10 +1,8 @@
 [tox]
 minversion = 3.18.0
 envlist = py3,pypy,pep8
-ignore_basepython_conflict = true
 
 [testenv]
-basepython = python3
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
@@ -70,7 +68,6 @@ commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [testenv:functional]
-basepython = python3
 setenv =
-    OS_TEST_PATH=./oslo_privsep/functional
-    OS_LOG_CAPTURE=1
+  OS_TEST_PATH=./oslo_privsep/functional
+  OS_LOG_CAPTURE=1