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: Iccc0284c725dc5dea18d1e30cc35bd185119535d

tox.ini

@@ -1,9 +1,14 @@
 [tox]
-minversion = 1.6
+minversion = 3.1.1
 envlist = py35,py27,pep8
 skipsdist = True
+# 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 =
@@ -24,31 +29,24 @@ commands =
   find . -type f -name "*.pyc" -delete
 
 [testenv:py27]
-# TODO(efried): Remove this once https://github.com/tox-dev/tox/issues/425 is fixed.
-basepython = python2.7
 commands =
   {[testenv]commands}
   stestr run {posargs}
   stestr slowest
 
 [testenv:py35]
-# TODO(efried): Remove this once https://github.com/tox-dev/tox/issues/425 is fixed.
-basepython = python3.5
 commands =
   {[testenv]commands}
   stestr run {posargs}
   stestr slowest
 
 [testenv:pep8]
-basepython = python3
 commands = flake8
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 # TODO(stephenfin): Remove the PYTHON hack below in favour of a [coverage]
 # section once we rely on coverage 4.3+
 #
@@ -66,14 +64,12 @@ commands =
   coverage report
 
 [testenv:docs]
-basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:bashate]
-basepython = python3
 commands =
   bash -c "ls devstack/*.sh | xargs bashate -v {posargs}"
 whitelist_externals = bash
@@ -87,7 +83,6 @@ exclude =  .venv,.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,build,t
 local-check-factory = nova_powervm.hacking.checks.factory
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt