Remove support for Python 3.8 and older

Python 3.6 and 3.7 are not supported any of active stable branches now,
so no longer need to be supported.

Also remove Python 3.8 support according to [1].

[1]
https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/thread/FOWV4UQZTH4DPDA67QDEROAESYU5Z3LE/

Also add a few new versions currently tested.

Change-Id: I2773938f4e7c4db213e9435fbf7ceaa6e0887f43
diff --git a/releasenotes/notes/drop-python38-support-8bfac4d85b4b19e2.yaml b/releasenotes/notes/drop-python38-support-8bfac4d85b4b19e2.yaml
new file mode 100644
index 0000000..f0ada81
--- /dev/null
+++ b/releasenotes/notes/drop-python38-support-8bfac4d85b4b19e2.yaml
@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Support for Python 3.8 has been dropped. Latest release of
+    ironic-tempest-plugin to support python 3.8 is 2.11.0.
+    The minimum version of Python now supported is Python 3.9.
diff --git a/setup.cfg b/setup.cfg
index a5cf426..c1cb2cf 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -6,6 +6,7 @@
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/ironic-tempest-plugin/latest/
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -14,10 +15,10 @@
     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 =