[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: I32d62e7721e8250c237cede6f284da3fe8367b33

tox.ini

@@ -14,7 +14,7 @@ install_command =
   pip install {opts} {packages}
 
 [testenv:py35]
-basepython = python3
+basepython = python3.5
 deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}
 
@@ -29,7 +29,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = ostestr {posargs}
 
 [testenv:pep8]
-basepython = python2.7
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} . unit_tests
 
@@ -61,6 +61,7 @@ omit =
     unit_tests/*
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]