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: I7ea4cb2da696cfa10551f4cab645db6d47de23d9

.zuul.yaml

@@ -4,5 +4,4 @@
       - openstack-cover-jobs-horizon
       - openstack-lower-constraints-jobs
       - openstack-python-jobs-horizon
-      - openstack-python35-jobs-horizon
       - openstack-python36-jobs-horizon

setup.cfg

@@ -15,7 +15,7 @@ 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

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,pep8,py35
+envlist = py27,pep8,py36
 minversion = 2.6
 skipsdist = True