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: Ibea0d909fe6628a387becc89bb58af18278d27c8

.zuul.yaml

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

tox.ini

@@ -25,10 +25,6 @@ deps = -r{toxinidir}/test-requirements.txt
 basepython = python3.4
 deps = -r{toxinidir}/test-requirements.txt
 
-[testenv:py35]
-basepython = python3.5
-deps = -r{toxinidir}/test-requirements.txt
-
 [testenv:py36]
 basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt