Sync Sphinx requirement

Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.

Change-Id: I7bbcd74da1ea5800db93e36eb78e028c6af7a061

test-requirements.txt

@@ -2,6 +2,7 @@ hacking>=1.1.0,<1.2.0 # Apache-2.0
 openstackdocstheme>=1.11.0  # Apache-2.0
 
 oslotest
-sphinx>=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,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
 testrepository>=0.0.18
 reno>=1.8.0 # Apache-2.0