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: I0d64734864c9d930059bf1a8251315a53c02069f

releasenotes/notes/remove-py39-a294c2d7335b646e.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

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/python-keystoneclient/latest/
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,7 +15,6 @@ 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