Remove Python 3.9 support

Python 3.9 is no longer part of the tested runtimes[1].

[1] https://governance.openstack.org/tc/reference/runtimes/2025.2.html

Change-Id: I86d7914841d632a3c4e6bc4b2a3ed4516dd94343

releasenotes/notes/remove-py39-4dcb7e79fa3d6f66.yaml

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

setup.cfg

@@ -5,7 +5,7 @@ description_file = README.rst
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/python-ironicclient/latest/
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -16,7 +16,6 @@ classifier =
     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