Remove support for Python 3.8

Change-Id: I68026e2e9bf98e6106d14c15e788e54e0869cf03
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

releasenotes/notes/remove-py38-895d36d36c85d0a2.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    Python 3.8 is no longer supported.

setup.cfg

@@ -6,7 +6,7 @@ summary = Refresh system configuration
 description_file =
     README.rst
 home_page = http://opendev.org/openstack/os-refresh-config
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Development Status :: 4 - Beta
     Environment :: Console
@@ -19,7 +19,6 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3 :: Only
-    Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11