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

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

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

All stable jobs need to run on py3 as master ironic-tempest-plugin
cannot support py2 anymore. Any py2 cloud/env can be tested by
installing the Temepest and plugins on py3 venv.

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: Ibea78605aedb955e3dc70252ca380c7e5f8f8726
diff --git a/tox.ini b/tox.ini
index 410337a..60be8ec 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,10 +1,11 @@
 [tox]
 minversion = 3.1.0
-envlist = py3,py27,pep8
+envlist = py3,pep8
 skipsdist = True
 ignore_basepython_conflict=true
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -15,29 +16,23 @@
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:pdf-docs]
-basepython = python3
 whitelist_externals = make
 commands = sphinx-build -b latex doc/source doc/build/pdf
            make -C doc/build/pdf
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]