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: I940661f000d558539caf9264a9dab1e9f55307f7

.zuul.yaml

@@ -1,5 +1,5 @@
 - project:
     templates:
       - python-charm-jobs
-      - openstack-python35-jobs
+      - openstack-python36-jobs
       - openstack-cover-jobs

tox.ini

@@ -3,7 +3,7 @@
 # within individual charm repos.
 [tox]
 skipsdist = True
-envlist = pep8,py34,py35
+envlist = pep8,py36
 skip_missing_interpreters = True
 
 [testenv]
@@ -33,18 +33,13 @@ deps =
 whitelist_externals = true
 commands = true
 
-[testenv:py34]
-basepython = python3.4
-deps = -r{toxinidir}/test-requirements.txt
-commands = stestr run {posargs}
-
-[testenv:py35]
-basepython = python3.5
+[testenv:py36]
+basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
 [testenv:pep8]
-basepython = python3.5
+basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} src unit_tests