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: Ia2c270d17a6189343c2ce345876b166e125f0682
Co-authored-by: Stephen Finucane <stephenfin@redhat.com>
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

releasenotes/notes/remove-python39.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

@@ -7,16 +7,16 @@ description_file = README.rst
 description_content_type = text/x-rst
 home_page = https://docs.openstack.org/freezer/latest/
 license = Apache-2.0
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Programming Language :: Python
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     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
     Development Status :: 5 - Production/Stable
     Natural Language :: English
     Environment :: OpenStack