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

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

designate-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: Ib3bd70ba8f2e8386643c536ac8194fa99b54b842
diff --git a/requirements.txt b/requirements.txt
index 2af96d6..8d0dc5f 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,8 +2,7 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 
-dnspython>=1.14.0;python_version=='2.7' # http://www.dnspython.org/LICENSE
-dnspython3!=1.13.0,!=1.14.0,>=1.12.0;python_version>='3.0' # http://www.dnspython.org/LICENSE
+dnspython3!=1.13.0,!=1.14.0,>=1.12.0 # http://www.dnspython.org/LICENSE
 ddt>=1.0.1 # MIT
 testtools>=2.2.0 # MIT
 tempest>=17.1.0 # Apache-2.0