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: I3fb435e6494239c2b8cc5a87c3b90dc96c31d006

releasenotes/notes/remove-py39-a3f911847e0075b6.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

@@ -7,7 +7,7 @@ license = Apache License, Version 2.0
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://opendev.org/openstack/python-observabilityclient
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Environment :: Console
     Environment :: OpenStack
@@ -20,7 +20,6 @@ classifier =
     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