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: I160c141169d68fd9403604c68cecde0c70c463c3

.zuul.yaml

@@ -5,7 +5,6 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
       - openstackclient-plugin-jobs

setup.cfg

@@ -17,7 +17,6 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Topic :: Internet :: Name Service (DNS)
 

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py27,flake8
+envlist = py36,py27,flake8
 minversion = 2.0
 skipsdist = True