Remove support for Pyton 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: I295e5d66ad826d70b0b8c9aab0514fc672066ff6

releasenotes/notes/drop-python38-support-68f7bfafaefc8d41.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Support for Python 3.8 has been dropped. Latest release of
+    cinder-tempest-plugin to support python 3.8 is 1.15.0.
+    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-discuss@lists.openstack.org
 url = http://www.openstack.org/
-python_requires = >=3.6
+python_requires = >=3.9
 classifiers =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,12 +15,10 @@ classifiers =
     Programming Language :: Python
     Programming Language :: Python :: 3 :: Only
     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 =