Sync Sphinx requirement
1. Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.
2. Remove unncessary "=="
Change-Id: I6988ed388cc6543bc49e62172aa93f34fdca8e95
diff --git a/doc/requirements.txt b/doc/requirements.txt
index d2e8cfc..ddf8411 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -3,7 +3,8 @@
# process, which may cause wedges in the gate later.
sphinxcontrib-apidoc>=0.2.0 # BSD
-sphinx>=1.6.2,!=1.6.6,!=1.6.7 # 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,!=2.1.0;python_version>='3.4' # BSD
openstackdocstheme>=1.18.1 # Apache-2.0
# releasenotes