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

This reverts commit e01117f538.

Keeping Python 3.10 in setup.cfg classifier and zuul.yaml changes.

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: Icd345455170170934d809b7e2643f7bd2dc7778a

setup.cfg

@@ -6,7 +6,7 @@ summary = Oslo Metrics API
 description_file =
     README.rst
 home_page = https://opendev.org/openstack/oslo.metrics
-python_requires = >=3.9
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Developers
@@ -15,6 +15,7 @@ classifier =
     Operating System :: OS Independent
     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