Dropping the py35 testing

All the integration testing has been moved to
Bionic now[1] and py3.5 is not tested runtime for
Train or stable/stein[2].

As per below ML thread, we are good to drop the py35
testing now:
http://lists.openstack.org/pipermail/openstack-discuss/2019-April/005097.html

[1] http://lists.openstack.org/pipermail/openstack-discuss/2019-April/004647.html
[2]
https://governance.openstack.org/tc/reference/runtimes/stein.html
https://governance.openstack.org/tc/reference/runtimes/train.html

Change-Id: Ic7555b73e986cdeaacd33fb40aff627c0ae380a4

.zuul.yaml

@@ -1,4 +1,4 @@
 - project:
     templates:
-      - python35-charm-jobs
+      - python36-charm-jobs
       - openstack-cover-jobs

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 skipsdist = True
-envlist = pep8,py35
+envlist = pep8,py36
 skip_missing_interpreters = True
 
 [testenv]
@@ -10,11 +10,6 @@ setenv = VIRTUAL_ENV={envdir}
 install_command =
   pip install {opts} {packages}
 
-[testenv:py35]
-basepython = python3
-deps = -r{toxinidir}/test-requirements.txt
-commands = ostestr {posargs}
-
 [testenv:py36]
 basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt