Blacklist sphinx 2.1.0 (autodoc bug)

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

Change-Id: I3adad072b1acacecec47be076b5fca67fd1a9469

Author: pengyuesheng

doc/requirements.txt

@@ -4,5 +4,5 @@
 
 openstackdocstheme>=1.18.1 # Apache-2.0
 sphinx>=1.6.2,!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
-sphinx>=1.6.2,!=1.6.6,!=1.6.7;python_version>='3.4' # BSD
+sphinx>=1.6.2,!=1.6.6,!=1.6.7,!=2.1.0;python_version>='3.4' # BSD
 reno>=2.5.0 # Apache-2.0