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: I50ff5b1b226ef974f08c489b74bd5e04b0cd5e12

releasenotes/notes/drop-python-3-8-2a8b6960c0ce7850.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.8 support was dropped. 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/heat-dashboard/latest/
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,7 +17,6 @@ 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