Declare Python 3.12 support

Now Python 3.12 unit tests job is voting. So we can safely say this
is tested with Python 3.12 .

Also bump the min python version because python <= 3.7 is no longer
supported by any maintained branches.

Change-Id: I50d69751aa61a9639453965fa1af3ba623abf4f0
diff --git a/setup.cfg b/setup.cfg
index 1b262e9..0dcbc7a 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 = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,11 +15,11 @@
     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
 
 keywords =
     setup