[ussuri][goal] Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

keystone-tempest-plugin is ready with python 3 and ok to drop the
python 2.7 support.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I4c46dfbfb9e679d64f6cc6b99d8f63775ec3914a
diff --git a/test-requirements.txt b/test-requirements.txt
index 327b0a4..4b13fd1 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -4,8 +4,7 @@
 
 hacking>=1.1.0,<1.2.0 # Apache-2.0
 
-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
+sphinx!=1.6.6,!=1.6.7,>=1.6.2  # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0
 
 # releasenotes