Use tox 3.1.1 and basepython fix

tox 3.1.1 fixed https://github.com/tox-dev/tox/issues/477 which
mishandled the basepython directive. Require this version and remove the
workarounds.

http://lists.openstack.org/pipermail/openstack-dev/2018-July/132085.html

Change-Id: Id7d904e42a87576c5e9b3fa34c7e2b036722e52d

tox.ini

@@ -1,9 +1,14 @@
 [tox]
-minversion = 1.6
+minversion = 3.1.1
 skipsdist = True
 envlist = py35,py27,pep8
+# Automatic envs (pyXX) will use the python version appropriate to that
+# env and ignore basepython inherited from [testenv]. That's what we
+# want, and we don't need to be warned about it.
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
@@ -17,23 +22,18 @@ whitelist_externals = bash
 commands = bash tools/pretty_tox.sh '{posargs}'
 
 [testenv:pep8]
-basepython = python3
 commands = flake8
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 commands = python -m ceilometer.openstack.common.lockutils python setup.py testr --slowest --coverage --testr-args="{posargs}"
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:bashate]
-basepython = python3
 commands =
   bash -c "ls devstack/*.sh | xargs bashate -v {posargs}"
 whitelist_externals = bash