Revert "Moves supported python runtimes from version 3.8 to 3.10"

This reverts commit 8535f84148.

Keeping Python 3.10 in setup.cfg classifier. 

Reason for revert:

Needed-By: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/882175

TC has been discussing about re-adding the python 3.8
testing in current master 2023.2 release testing.

- https://meetings.opendev.org/meetings/tc/2023/tc.2023-04-25-18.00.log.html#l-191
- https://lists.openstack.org/pipermail/openstack-discuss/2023-April/033469.html

While governance changes are under review, TC agreed to add py3.8 testing
so that we do not see more project/lib dropping python 3.8 and make them
uninstalable on python 3.8

- https://meetings.opendev.org/meetings/tc/2023/tc.2023-05-02-18.00.log.html#l-17
- https://review.opendev.org/c/openstack/governance/+/882165

Also adding py3.8 testing back in job https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/882175

Change-Id: Ie1d0a1ce0439ca5ffe3d3d377f3ac14a4fa6702f

setup.cfg

@@ -7,7 +7,7 @@ author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/taskflow/latest/
 keywords = reliable,tasks,execution,parallel,dataflow,workflows,distributed
-python_requires = >=3.9
+python_requires = >=3.8
 classifier =
     Development Status :: 4 - Beta
     Environment :: OpenStack
@@ -17,6 +17,7 @@ classifier =
     Operating System :: POSIX :: Linux
     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 :: Only