Remove Python 3.8 support

Python 3.8 is no longer part of tested runtimes since 2024.2 . Removing
support for Python 3.8 allows us to replace deprecated md5 wrapper from
oslo.utils [1] by direct call of hashlib.md5.

[1] https://review.opendev.org/c/openstack/oslo.utils/+/930879

Also add python 3.12 to classifiers because now py312 unit test job is
voting.

Change-Id: I53da305538e27f2ff20a1ecb25960ebb03388011

setup.cfg

@@ -10,7 +10,7 @@ project_urls =
     Bug Tracker = https://bugs.launchpad.net/nova/
     Documentation = https://docs.openstack.org/nova/
     Source Code = https://opendev.org/openstack/nova
-python_requires = >=3.8
+python_requires = >=3.9
 classifiers =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -20,10 +20,10 @@ classifiers =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     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
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython