Support back python 3.5

While dropping py2 support, we dropped py3.5 support also
- I1ced6583d61418ca5cd2deb72b7e3f7a3ec986e2

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: Ie8af65c8aef92c4193071133bc3a4df98725da68

.zuul.yaml

@@ -2,4 +2,5 @@
     templates:
       - check-requirements
       - publish-openstack-docs-pti
+      - openstack-python35-jobs
       - openstack-python3-ussuri-jobs

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/os-performance-tools/latest/
-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
     Programming Language :: Python :: 3 :: Only