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: I5ed40f7a0aea4f4d6bbfa6723718f4dd6aeb61db

releasenotes/notes/drop-python38-support-6c2833c61d2fa750.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 3.8 support has been dropped. Last release of
+    barbican-tempest-plugin to support python 3.8 is 4.1.0.
+    The minimum version of Python now supported is Python 3.9.

setup.cfg

@@ -5,7 +5,7 @@ long_description = file: README.rst
 author = OpenStack
 author_email = openstack-dev@lists.openstack.org
 url = http://www.openstack.org/
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -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 =