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: I17d4ecbe471032b256b2113768a8e62c4fd45161

.zuul.yaml

@@ -1,3 +1,3 @@
 - project:
     templates:
-      - python35-charm-jobs
+      - python36-charm-jobs

tox.ini

@@ -2,7 +2,7 @@
 # This file is managed centrally by release-tools and should not be modified
 # within individual charm repos.
 [tox]
-envlist = pep8,py35,py36
+envlist = pep8,py36
 skipsdist = True
 
 [testenv]
@@ -22,11 +22,6 @@ basepython = python2.7
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
-[testenv:py35]
-basepython = python3.5
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
-
 [testenv:py36]
 basepython = python3.6
 deps = -r{toxinidir}/requirements.txt