Fix tox setup

Use proper combination of tox settings so that we install our sources
and not install the latest release of openstackdocstheme from pip since
it's a dependency of os-api-ref.

This works around a regression in tox/virtualenv/setuptools.

Co-Authored-By: Jeremy Stanley <fungi@yuggoth.org>
Change-Id: I6b3bf39d517a757cfc7240c770f4da67cccc0a3e

tox.ini

@@ -1,9 +1,9 @@
 [tox]
-minversion = 1.6
+minversion = 2.0
 envlist = pep8
 
 [testenv]
-usedevelop = True
+usedevelop = False
 install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
 setenv = VIRTUAL_ENV={envdir}