[ussuri][goal] Updates for python 2.7 drop

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

charm repos need few updates on either py2 drop or updating
ussuri python template, tox.ini etc.

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: I9eaa8b3baaf84f22194adc4de8feaab77e136bf5

.zuul.yaml

@@ -2,4 +2,4 @@
     templates:
       - python35-charm-jobs
       - openstack-cover-jobs
-      - openstack-python3-train-jobs
+      - openstack-python3-ussuri-jobs

tox.ini

@@ -2,10 +2,13 @@
 # This file is managed centrally by release-tools and should not be modified
 # within individual charm repos.
 [tox]
+minversion = 3.1
 skipsdist = True
 envlist = pep8,py37
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
          TERM=linux
@@ -18,12 +21,10 @@ deps =
     -r{toxinidir}/requirements.txt
 
 [testenv:build]
-basepython = python3
 commands =
     charm-build --log-level DEBUG -o {toxinidir}/build src {posargs}
 
 [testenv:py3]
-basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
@@ -43,14 +44,12 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
 [testenv:pep8]
-basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} src unit_tests
 
 [testenv:cover]
 # Technique based heavily upon
 # https://github.com/openstack/nova/blob/master/tox.ini
-basepython = python3
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 setenv =
@@ -76,7 +75,6 @@ omit =
     unit_tests/*
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [flake8]