Update sphinx requirements

Following recent changes to global requirements [1], we need
to update sphinx requirements locally.

[1] If558f184c959e4b63b56dec3ca1571d1034cfe5c

Change-Id: I9bd0ab95e425fbfd108f4805d3a223147591d822
diff --git a/test-requirements.txt b/test-requirements.txt
index 698b5f9..b5667d2 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -4,7 +4,8 @@
 
 hacking!=0.13.0,<0.14,>=0.12.0 # 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
 
 reno>=2.5.0 # Apache-2.0