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@lists.openstack.org/thread/FOWV4UQZTH4DPDA67QDEROAESYU5Z3LE/

Change-Id: I07bf5d9073dc077992866afe102abb5b5a1e4b9f

releasenotes/notes/drop-python38-support-81f678e8740165ac.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 3.8 support has been dropped. Last release of
+    vitrage-tempest-plugin to support python 3.8 is 6.4.0.
+    The minimum version of Python now supported is Python 3.9.

setup.cfg

@@ -14,10 +14,11 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     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 =