Support py3.12 and drop py3.8

As per the current release tested runtime, we test
from python 3.9 till python 3.12 so updating the same
in python classifier in setup.cfg

Also, dropping the python3.8 support
- https://lists.openstack.org/archives/list/openstack-discuss@li>

Change-Id: I370490bc405c2a81a4822e6bd1d86dfee5fc6e60

releasenotes/notes/drop-python38-support-70bd303e0defb9e1.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 3.8 support has been dropped. Last release of
+    cloudkitty-tempest-plugin to support python 3.8 is 3.0.0.
+    The minimum version of Python now supported is Python 3.9.

setup.cfg

@@ -14,10 +14,10 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11
+    Programming Language :: Python :: 3.12
 
 [files]
 packages =