finalize python 3.9 support removal

The last release of openstack to support python 3.9
was 2025.1 (epoxy), with this change watcher now requires
3.10, testing of 3.9 was removed in previous commits.

Change-Id: Ida53740293e93b0c20dec2e175b390fa18bed852
Signed-off-by: Sean Mooney <work@seanmooney.info>

releasenotes/notes/drop-py39-8a9c99678b3e8eeb.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Watcher now requires python 3.10 or newer.
+    The last release of watcher to support ``3.9`` was ``2025.1``.
+    Please ensure you have a supported python version before upgrading.

setup.cfg

@@ -6,8 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/watcher/latest/
-# TODO(sean-k-mooney): bump to >= 3.10 before m3.
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology