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

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

neutron-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

Depends-On: https://review.opendev.org/#/c/705184/

Change-Id: I9daafef0847a9a9baa45382b87da71fe8d1fffd0
diff --git a/tox.ini b/tox.ini
index 95352a2..19e006a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
+minversion = 3.1
 envlist = pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}
@@ -19,7 +21,6 @@
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands =
   sh ./tools/misc-sanity-checks.sh
   flake8
@@ -27,11 +28,9 @@
   sh
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:cover]
-basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source neutron_tempest_plugin --parallel-mode
@@ -42,16 +41,13 @@
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:releasenotes]
-basepython = python3
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
-basepython = python3
 commands = oslo_debug_helper -t neutron_tempest_plugin/ {posargs}
 
 [flake8]