Update python versions, drop py3.8

The current testing runtime [1] states testing from py3.9
to 3.12. The patch updates setup.cfg to reflect the correct
python versions.

The patch also drops python 3.8 support following [2].

[1] https://governance.openstack.org/tc/reference/runtimes/2025.1.html
[2] https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/thread/FOWV4UQZTH4DPDA67QDEROAESYU5Z3LE/

Change-Id: I2d13409c9bfffc866e31af52611a26f6037021cc

releasenotes/notes/drop-python38-support-eeb19a0bc0160sw1.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 3.8 support has been dropped. Last release of watcher
+    supporting python 3.8 is 13.0.0.
+    The minimum version of Python now supported is Python 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/watcher/latest/
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,10 +17,10 @@ classifier =
     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
 
 [files]
 packages =