Update Python runtime for 2024.2

Min python version is 3.9[1]. Use py3 to reduce need for changes in
future.

[1] https://governance.openstack.org/tc/reference/runtimes/2024.2.html

Change-Id: I46bdf025f5ae495881f450f34844105753e9058d

Author: waipeng

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.18
-envlist = py38,pep8
+envlist = py3,pep8
 ignore_basepython_conflict = True
 
 [testenv]