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: I288e1b8d0fcbea31826b85a1ee1b2aad670ff0a2
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

releasenotes/notes/remove-py39-16ed26d245366f13.yaml

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

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/etcd3gw/latest/
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Environment :: Console
     Environment :: OpenStack
@@ -16,10 +16,10 @@ classifier =
     Operating System :: OS Independent
     Programming Language :: Python
     Programming Language :: Python :: 3
-    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
     Topic :: System :: Distributed Computing