Drop support for Python 3.9

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

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

Change-Id: Ica9c91fd9059b728e359740e6589613b13fa592e
Co-authored-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

releasenotes/notes/drop-python-39.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    Support for Python 3.9 has been dropped.

setup.cfg

@@ -3,15 +3,16 @@ name = python-mistralclient
 summary = Mistral Client Library
 description_file = README.rst
 license = Apache Software License
-python_requires = >=3.9
+python_requires = >=3.10
 classifiers =
     Programming Language :: Python
     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
+    Programming Language :: Python :: 3.13
     Environment :: OpenStack
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators
@@ -115,7 +116,6 @@ openstack.workflow_engine.v2 =
     dynamic_action_delete = mistralclient.commands.v2.dynamic_actions:Delete
     dynamic_action_update = mistralclient.commands.v2.dynamic_actions:Update
 
-
 mistralclient.auth =
     # Standard Keystone authentication.
     keystone = mistralclient.auth.keystone:KeystoneAuthHandler