Bump minimum python version

Python 3.8 and older versions have been untested for several cycles.
For example python 3.8 was removed from tested runtimes in 2024.2 and
has been untested since then.

Change-Id: Ifdec3eae09e7a281bf20520a2ddb95a8c64d0832

releasenotes/notes/remove-py38-49274651f8385f4a.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Support for Python 3.8 and older versions has been removed. Now the minimum
+    python version supported is 3.9 .

setup.cfg

@@ -6,7 +6,7 @@ author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://opendev.org/openstack/metalsmith
 license = Apache-2
-python_requires = >=3.6
+python_requires = >=3.9
 classifier =
     Environment :: Console
     Environment :: OpenStack
@@ -18,10 +18,10 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
-    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
 
 [files]
 packages =
@@ -38,4 +38,4 @@ console_scripts =
 [codespell]
 quiet-level = 4
 # Words to ignore:
-# ignore-words-list = example
+# ignore-words-list = example