Update sphinx dependency
Sphinx 2.0 no longer works on python 2.7, so we need to start capping
it there as well.
# be installed in a specific order.
openstackdocstheme>=1.18.1 # Apache-2.0
reno>=2.5.0 # Apache-2.0
-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
# The below is rewquired to build testing module reference
mock>=2.0.0 # BSD
