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: I8fbcde0573eddc1763104e0f812f93489a1b48e3
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

.zuul.yaml

@@ -22,7 +22,6 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
       - publish-openstack-docs-pti

tox.ini

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