Blacklist sphinx 2.1.0 (autodoc bug)

See https://github.com/sphinx-doc/sphinx/issues/6440 for upstream details
Depend-On: https://review.opendev.org/#/c/663060/

Change-Id: I648dd4d4a98a3d423c98d28c193737cffe1a2c49

doc/requirements.txt

@@ -4,7 +4,7 @@
 
 openstackdocstheme>=1.18.1 # Apache-2.0
 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
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4' # BSD
 reno>=2.5.0 # Apache-2.0
 
 # imported when the source code is parsed for generating documentation: