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: I6767a26645b8139b60a0ca7960ca4ba391eecc09

releasenotes/notes/remove-py39-88ae5d7e3cf12f7d.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-cinderclient/latest/
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: Console
@@ -18,7 +18,6 @@ classifier =
     Programming Language :: Python
     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