Update python testing as per zed cycle teting runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Moving the py36 job to py38 based as well as
updating the python classifier also to reflect the same.

[1] https://governance.openstack.org/tc/reference/runtimes/zed.html

Change-Id: I9b3a05c708d53d1e7775eefdac802392fd18bc24

releasenotes/notes/drop-python-3-6-and-3-7-0b299b4dc9673c6e.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.6 & 3.7 support has been dropped. The minimum version of Python now
+    supported is Python 3.8.