Cap sphinx for py2 to match global requirements

Sphinx 2.0.0 dropped Python 2.7 support. This is aligned with
[1] in requirements project.
[1]Change-Id If558f184c959e4b63b56dec3ca1571d1034cfe5c

Change-Id: Idedb6c23883844d5bf93f3ad2c20eaaeeac2162f

doc/requirements.txt

@@ -1,7 +1,8 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-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
 sphinxcontrib-actdiag>=0.8.5 # BSD
 sphinxcontrib-seqdiag>=0.8.4 # BSD
 sphinx-feature-classification>=0.2.0 # Apache-2.0