Remove Python 3.8 support

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

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

Change-Id: I5268bd8c0aedfb9711b5c5164958e42b96ca2b47

releasenotes/notes/remove-py38-bb1062b0726bf422.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

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/oslo.cache/latest
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -16,7 +16,6 @@ classifier =
     Programming Language :: Python
     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