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. Also
correct setup.cfg and tox.ini to reflect the current supported Python
versions.

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

Change-Id: I67c892e0b5afaf8e603a77102e06567a1bfe2eb6

.zuul.yaml

@@ -20,7 +20,6 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
       - publish-openstack-docs-pti

setup.cfg

@@ -16,8 +16,8 @@ classifier =
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = py36,py35,py27,pep8,releasenotes
+envlist = py37,py36,py27,pep8,releasenotes
 
 [testenv]
 usedevelop = True