Fix doc build

* Ensure doc is built with python3
* Ensures we don't install sphinx outside of doc build

Fixes sphinx requiring python3 issue

Change-Id: I2ef4c3c02e6af8c8b9891b39b59eec1a73c9a1dd
(cherry picked from commit efb015c143)

doc/requirements.txt

@@ -0,0 +1,5 @@
+# Even if we do not build doc with python 2, we have to
+# push that content due to some global check.
+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
+openstackdocstheme>=1.18.1 # Apache-2.0

test-requirements.txt

@@ -6,12 +6,10 @@ hacking>=0.12.0,!=0.13.0,<0.14  # Apache-2.0
 
 coverage>=4.0,!=4.4  # Apache-2.0
 python-subunit>=0.0.18 # Apache-2.0/BSD
-sphinx>=1.6.2  # BSD
 oslotest>=1.10.0 # Apache-2.0
 testrepository>=0.0.18  # Apache-2.0/BSD
 testscenarios>=0.4  # Apache-2.0/BSD
 testtools>=1.4.0 # MIT
-openstackdocstheme>=1.18.1 # Apache-2.0
 
 # releasenotes
 reno>=1.8.0 # Apache-2.0

tox.ini

@@ -30,10 +30,13 @@ commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
 basepython = python3
+deps = -r{toxinidir}/doc/requirements.txt
+       -r{toxinidir}/test-requirements.txt
 commands = python setup.py build_sphinx
 
 [testenv:releasenotes]
 basepython = python3
+deps = -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html