update sphinx requirement

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

Change-Id: Ia4ad99c8201f48292c35e7b50604d1d87c467eb3
(cherry picked from commit c3a23500ce)

doc/requirements.txt

@@ -1,7 +1,8 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-sphinx!=1.6.6,!=1.6.7,>=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
 sphinxcontrib-apidoc>=0.2.0  # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0
 os-api-ref>=1.4.0 # Apache-2.0