[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: Ic70ddd43786d056c35917caad6518a193d3586dd

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27
+envlist = pep8,py35
 skipsdist = True
 # NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
 sitepackages = False
@@ -14,8 +14,8 @@ install_command =
 whitelist_externals = true
 commands = true
 
-[testenv:py27]
-basepython = python2.7
+[testenv:py35]
+basepython = python3.5
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]