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: I58b7bafe53ae9fd23314ec7aab03a0029dd5e4ac

releasenotes/notes/remove-py38-16eda9eaf9fe00d0.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 @@ summary = Oslo Metrics API
 description_file =
     README.rst
 home_page = https://opendev.org/openstack/oslo.metrics
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Developers
@@ -15,7 +15,6 @@ classifier =
     Operating System :: OS Independent
     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