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: I0db6c9cbf2af9ed9c701755a26f721aeac97d4cd

tox.ini

 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = py36,py35,py27,pep8,pypy
+envlist = py36,py27,pep8,pypy
 
 [testenv]
 usedevelop=True

zuul.d/project.yaml

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