Blacklist sphinx 2.1.0 (autodoc bug)

See https://github.com/sphinx-doc/sphinx/issues/6440 for upstream details

Change-Id: I9a167c082d685f02766349ba9e301afd52d0d284

doc/requirements.txt

@@ -4,6 +4,6 @@
 openstackdocstheme>=1.18.1 # Apache-2.0
 osprofiler>=1.4.0 # Apache-2.0
 os-api-ref>=1.4.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4' # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
 sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7' # BSD
 reno>=2.5.0 # Apache-2.0

test-requirements.txt

@@ -17,7 +17,7 @@ osprofiler>=1.4.0 # Apache-2.0
 os-testr>=1.0.0 # Apache-2.0
 python-subunit>=1.0.0 # Apache-2.0/BSD
 pytz>=2013.6 # MIT
-sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4' # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
 sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7' # BSD
 testrepository>=0.0.18 # Apache-2.0/BSD
 stestr>=2.0.0 # Apache-2.0