Move to python3.9 as minimal python version

Python 3.8 goes EOL in October and since it's been dropped
for keystoneauth [1] recently, glance_store keytone-tips periodic
job started failing. Removing py3.8 support from testing runtime
for tox and updating python3.9 as minimal python version.

[1]: https://review.opendev.org/c/openstack/keystoneauth/+/928833

Change-Id: Iba2278340a227974792e5c9305e0d149ed7a030b

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/glance_store/latest/
-python_requires = >=3.8
+python_requires = >=3.9
 classifier =
     Development Status :: 5 - Production/Stable
     Environment :: OpenStack
@@ -18,7 +18,6 @@ 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

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py39,py38,pep8
+envlist = py39,pep8
 ignore_basepython_conflict = True
 
 [testenv]