Update Sphinx requirements and actually use Python 3 for docs

This fixes building the guidelines (again) since Sphinx 2.0.0 is not
compatible with Python 2.7, and the way we use basepython in tox.ini
is not correct (resulting in Python 2 being used by default).

Change-Id: Ieeeb319caa6d7a8dad78857c7dce105da33d2dd2

requirements.txt

@@ -3,7 +3,8 @@
 # process, which may cause wedges in the gate later.
 pbr>=1.0
 openstackdocstheme>=1.18.1 # Apache-2.0
-sphinx!=1.6.6,>=1.6.2 # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7'  # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4'  # BSD
 stestr>=2.0.0 # Apache-2.0
 testtools>=0.9.36,!=1.2.0
 yasfb>=0.8.0

tox.ini

@@ -4,9 +4,9 @@
 minversion = 1.4
 envlist = docs,py37
 skipsdist = True
-basepython = python3
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}