Drop 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: I0aa0fd95fd882ab0989eed81d535b963c821c162

.zuul.yaml

@@ -3,7 +3,6 @@
       - check-requirements
       - release-notes-jobs-python3
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
       - publish-openstack-docs-pti
       - openstack-lower-constraints-jobs

setup.cfg

@@ -16,7 +16,7 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py3,py27,pep8
+envlist = py36,py27,pep8
 skipsdist = True
 
 [testenv]