Merge "Update sphinx requirement."

Author: Zuul

doc/requirements.txt

@@ -3,5 +3,6 @@
 # process, which may cause wedges in the gate later.
 openstackdocstheme>=1.23.2 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.5 # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.5,<2.0.0;python_version=='2.7' # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.5;python_version>='3.4' # BSD
 sphinxcontrib-apidoc>=0.2.0 # BSD