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 Python 3.13 which has been tested for some time.

Change-Id: I30b42eea4220dba7a890c1ec6db1d57ce3061a9b
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

releasenotes/notes/remove-py39-c95dedb7fe1cd572.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

@@ -6,17 +6,16 @@ author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/automaton/latest/
 description_file =
     README.rst
-python_requires = >=3.8
+python_requires = >=3.10
 classifier =
     Intended Audience :: Developers
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX
     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
     Topic :: Software Development :: Libraries