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

@@ -1,35 +1,29 @@
 [tox]
 envlist = py3,pep8
-ignore_basepython_conflict = True
-minversion = 2.0
+minversion = 3.18.0
 
 [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
-       -r{toxinidir}/test-requirements.txt
+  -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
 
 [testenv:cover]
 setenv =
-    PYTHON=coverage run --source os_collect_config --parallel-mode
+  PYTHON=coverage run --source os_collect_config --parallel-mode
 commands =
-    coverage erase
-    stestr run {posargs}
-    coverage combine
-    coverage html -d cover
-    coverage xml -o cover/coverage.xml
-    coverage report
+  coverage erase
+  stestr run {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
+  coverage report
 
 [testenv:venv]
 commands = {posargs}