Remove Python 3.8 support

Python 3.8 is no longer part of the tested runtimes since 2024.2[1]
because its EOL is coming soon.

Also officially declare python 3.12 support because python 3.12 is now
part of tested runtimes and unit test job with Python 3.12 is now
voting.

[1] https://governance.openstack.org/tc/reference/runtimes/2024.2.html

Change-Id: I472c27133b59c18e08d60f112bb6a77c8bc9d4a8

releasenotes/notes/remove-py38-80670bdcfd4dd135.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.8 support was dropped. The minimum version of Python now supported
+    is Python 3.9.

setup.cfg

@@ -7,7 +7,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/ceilometer/latest/
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -18,10 +18,10 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11
+    Programming Language :: Python :: 3.12
     Topic :: System :: Monitoring
 
 [files]