Remove Python 3.8 support

Python 3.8 was removed from the tested runtimes for 2024.2[1] and has
not been tested since then.

Also add Python 3.12 which is part of the tested runtimes for 2025.1.
Now unit tests job with Python 3.12 is voting.

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

Change-Id: Ic7a7b70c5d7491c6fc095c859c21fb9dd56bfc39

releasenotes/notes/remove-py38-ecd3b5c9b6799e75.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Support for Python 3.8 has been removed. Now the minimum python version
+    supported is 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/barbican/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
 
 [files]
 data_files =