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

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

ec2api-tempest-plugin is ready with python 3 and ok to drop the
python 2.7 support.

Removing the USE_PYTHON3=True from base job so that it can
keep running  on py3 for < ussuri branches.

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: I49feaa3eddf66058ebd061b76b76172cb4373c3e

.zuul.yaml

@@ -1,6 +1,5 @@
 - project:
     templates:
-      - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python3-ussuri-jobs
       - check-requirements

tox.ini

@@ -1,7 +1,8 @@
 [tox]
-minversion = 2.0
+minversion = 3.1.1
 envlist = py37,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True