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: I2e38f54ad658b670d4f8f66f14b45eced6d5b7dd

.zuul.yaml

@@ -3,7 +3,6 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
       - publish-openstack-docs-pti

setup.cfg

@@ -11,7 +11,7 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
     Development Status :: 5 - Production/Stable
     Natural Language :: English
     Environment :: OpenStack

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{36,35,27},pep8,pylint,docs
+envlist = py{36,27},pep8,pylint,docs
 skipsdist = True
 
 [testenv]