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: I2b2b54c9f54f55002b627bc9e29a65d841fbc29c
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

releasenotes/notes/remove-py39-b6f3ce23fc9ea9b9.yaml

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

setup.cfg

@@ -7,7 +7,7 @@ author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/taskflow/latest/
 keywords = reliable,tasks,execution,parallel,dataflow,workflows,distributed
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -17,10 +17,10 @@ classifier =
     Operating System :: POSIX :: Linux
     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
     Programming Language :: Python :: Implementation :: CPython
     Topic :: Software Development :: Libraries