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: Id0afb54c524044f6c0cde11a97b809a392add73f

releasenotes/notes/remove-py39-09c21d2e8ba2200f.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Support for Pyton 3.9 has been removed. Now Python 3.10 is the minimum
+    version supported.

setup.cfg

@@ -8,7 +8,7 @@ license = Apache-2.0
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/python-heatclient/latest
-python_requires = >=3.9
+python_requires = >=3.10
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: Console
@@ -18,7 +18,6 @@ classifier =
     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