Drop py35 jobs

Python 3.5 was the target runtime for the Rocky release. The current
target py3 runtime for Stein is Python 3.6, so there is no reason to
keep testing against the older version.

https://governance.openstack.org/tc/reference/runtimes/stein.html#python-runtime-for-stein

Change-Id: Ia99ea817fc1199a937a163610e1860897be316be

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = pep8,py37,py36,py35,py27
+envlist = pep8,py37,py36,py27
 skipsdist = True
 
 [testenv]

zuul.d/layout.yaml

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