Remove py35 tox jobs

At this point py35 practically means Xenial, and we are not interested
in having that as a build platform.  Drop the py35 jobs.

Update python setup.cfg metadata; we are building on 3.9 so add that.

Change-Id: I981f0f67a6fd809af1ab70934358dc3404890f35

.zuul.d/project.yaml

@@ -7,7 +7,6 @@
       # NOTE(ianw): 2020-05-29 jobs are unstable; see
       # https://bugs.launchpad.net/tripleo/+bug/1879766
       - tripleo-buildimage-jobs-centos-8
-      - openstack-python35-jobs
       - openstack-python3-wallaby-jobs
     check:
       jobs:

setup.cfg

@@ -12,12 +12,12 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Environment :: Console
-python_requires = >=3.5
+python_requires = >=3.6
 
 [files]
 packages =