Remove support for Python 3.8 and 3.9

Python 3.9 is no longer part of the tested runtimes [1]. Python 3.8
testing was removed in the 2024.2 release [2].

Add Python 3.12 to classifiers.

[1] https://governance.openstack.org/tc/reference/runtimes/2025.2.html
[2] https://governance.openstack.org/tc/reference/runtimes/2024.2.html

Change-Id: I3299b9733ba9e79d51c4f603ad0d0a90efecbbab

releasenotes/notes/drop-python-3-8-and-3-9-51d2e25d71240731.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.8 and 3.9 support has been dropped. The minimum version of Python
+    now supported is Python 3.10.

setup.cfg

@@ -3,16 +3,15 @@ name = python-blazarclient
 summary = Client for OpenStack Reservation Service
 description_file = README.rst
 license = Apache Software License
-python_requires = >=3.8
+python_requires = >=3.10
 classifiers =
     Programming Language :: Python
     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
     Environment :: OpenStack
     Development Status :: 3 - Alpha
     Framework :: Setuptools Plugin