Remove python 3.9 support

OpenStack recently dropped python3.9 support for global requirements[1]
as it was removed from the Flamingo supported runtimes[2].

So this patch removes 3.9 support from nova too.

[1]https://review.opendev.org/c/openstack/requirements/+/948285
[2]https://governance.openstack.org/tc/reference/runtimes/2025.2.html#python

Change-Id: I8aea971d7972959c32d5175926cbaddb21839f8e

setup.cfg

@@ -10,7 +10,7 @@ project_urls =
     Bug Tracker = https://bugs.launchpad.net/nova/
     Documentation = https://docs.openstack.org/nova/
     Source Code = https://opendev.org/openstack/nova
-python_requires = >=3.9
+python_requires = >=3.10
 classifiers =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -20,7 +20,6 @@ classifiers =
     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