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: I579dea229152bde1178f06eab75bc19a44c474fd

.zuul.yaml

@@ -4,7 +4,6 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
       - release-notes-jobs-python3

setup.cfg

@@ -12,7 +12,7 @@ classifier=
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
 keywords = openstack monitoring email
 description-file = README.rst
 home-page = https://github.com/openstack/monasca-notification

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,35,36,py},pep8,cover
+envlist = py{27,36,py},pep8,cover
 minversion = 2.7
 skipsdist = True