Refresh tox

Add ignore_basepython_conflict [1], this will fix tox warnings about
python versions.
Bump tox minversion to 3.1.0 to ensure that ignore_basepython_conflict
is supported.
Use the default install_commandi [2].

[1] https://tox.readthedocs.io/en/latest/config.html#conf-ignore_basepython_conflict
[2] https://tox.readthedocs.io/en/latest/config.html#conf-install_command

Change-Id: Ic76f25bcd2d70525d02c565449da753c4ba58bc3

tox.ini

@@ -1,14 +1,14 @@
 [tox]
-minversion = 2.0
+minversion = 3.1.0
 skipsdist = True
 envlist = py{36,37},pep8
+ignore_basepython_conflict=true
 
 [testenv]
 basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
        -r{toxinidir}/requirements.txt
        -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
-install_command = pip install {opts} {packages}
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
          CEILOMETER_TEST_BACKEND={env:CEILOMETER_TEST_BACKEND:none}