Drop Python 3.9 support

It was removed from tested runtimes in 2025.2 . It has been kept for
one cycle for smooth transition of the other projects, but it's time
to bump the enforced minimum requirement.

Also add recent python versions which have been tested for some time.

Change-Id: I8a2e4d71686b7cfb883637cb48691d22f862a8a3
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

releasenotes/notes/remove-py39-2d2966941e7ef634.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Support for Python 3.8 and 3.9 has been removed. Now the minimum python
+    version supported is 3.10.

setup.cfg

@@ -5,7 +5,7 @@ description_file = README.rst
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = http://www.openstack.org/
-python_requires = >=3.8
+python_requires = >=3.10
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -13,10 +13,10 @@ 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
+    Programming Language :: Python :: 3.13
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython