Support python 3.5

While dropping py2 support, we dropped py3.5 support also
- Ib769d9cbb7ba9f191014c5445ed7f289e013aef4

There might be some user of it using on py3.5 distro like
stable/rocky jobs on xenial.

There is no deps issue to support the py3.5 so let's keep
it as long as we can.

Change-Id: Ifa1e53fc8c1a665c9fb35845960cc8bc700d19c8

.zuul.d/jobs.yaml

@@ -1,5 +1,6 @@
 - project:
     templates:
+      - openstack-python35-jobs
       - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = http://docs.openstack.org/developer/bashate/
-python-requires = >=3.6
+python-requires = >=3.5
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,6 +15,7 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7