Disable auto-discovery for setuptools and update python testing

With setuptools release 61.0.0 docs build started to fail:
error: Multiple top-level packages discovered in a flat-layout:
['specs', 'attic', 'superseded'].

This bug is mentioned in setuptools issue 3197 [0], and the suggested
workaround is to disable auto-discovery by adding 'py_modules=[]' in
setup.py.

Also use recent python versions because the old ones are no longer available.

These 2 unrelated changes need to be merged together in order to unblock
the gate.

[0] https://github.com/pypa/setuptools/issues/3197

Change-Id: Iddc30b9521b61d9083c2b1f6e8a6707196ea0a57

.zuul.yaml

@@ -1,4 +1,4 @@
 - project:
     templates:
       - openstack-specs-jobs
-      - openstack-python3-ussuri-jobs
+      - openstack-python3-zed-jobs

setup.py

@@ -18,4 +18,5 @@ import setuptools
 
 setuptools.setup(
     setup_requires=['pbr'],
-    pbr=True)
+    pbr=True,
+    py_modules=[])