[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: Iea382a3ec4564d3bf00d16dda281673bc10cb33b

tox.ini

@@ -1,25 +1,21 @@
 [tox]
-envlist = pep8,py27,py34,py35
+minversion = 3.1
+envlist = pep8,py34,py35
 skipsdist = True
 # NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
 sitepackages = False
 # NOTE(beisner): Avoid false positives by not skipping missing interpreters.
 skip_missing_interpreters = False
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 install_command =
   pip install {opts} {packages}
 commands = ostestr {posargs}
 
-[testenv:py27]
-basepython = python2.7
-deps = -r{toxinidir}/test-requirements.txt
-# TODO: Need to write unit tests then remove the following command.
-# https://github.com/juju/charm-tools/issues/249
-commands = /bin/true
-
 [testenv:py34]
 basepython = python3.4
 deps = -r{toxinidir}/test-requirements.txt
@@ -35,12 +31,10 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = /bin/true
 
 [testenv:pep8]
-basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [flake8]