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: I45748bcdd44b1b7c209583ac78756680320ec755
diff --git a/setup.cfg b/setup.cfg
index 0dcbc7a..17775af 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -6,7 +6,7 @@
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = http://docs.openstack.org/developer/heat-tempest-plugin
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,7 +15,6 @@
     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