Update sphinx dependency
Sphinx 2.0 no longer works on python 2.7,
so we need to start capping it there as well
depend on
openstackdocstheme>=1.18.1 # Apache-2.0
reno>=2.5.0 # Apache-2.0
doc8>=0.6.0 # 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,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
mock>=2.0.0 # BSD
os-api-ref>=1.4.0 # Apache-2.0
ddt>=1.0.1 # MIT
