Fix sphinx requirement for python2

Ensure the sphinx requirement passes the requirements-check job.

Change-Id: I4f76e167a9d29e36dd66ecb82b92b63a3cd07d35
diff --git a/test-requirements.txt b/test-requirements.txt
index 6e99fe7..327b0a4 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -4,7 +4,8 @@
 
 hacking>=1.1.0,<1.2.0 # 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
 openstackdocstheme>=1.18.1 # Apache-2.0
 
 # releasenotes