Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.
Change-Id: Ia2fdfd2e99f0528c1322c08a745b651fbf5b1f73
# This is required for the docs build jobs
-sphinx # 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
openstackdocstheme # Apache-2.0
# This is required for the releasenotes build jobs