Add py38 as a runtime in tox.ini
Add py38 as a runtime in tox.ini

Drop py37 and add py38 for the latest python runtimes [1], although it
had to be updated in victoria.

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

Change-Id: I48642a703d6d1fe8be3055ee5a48eaafd0a18067
Signed-off-by: Yasufumi Ogawa <0cbdead7c4bf8da8f6bef3a7960478ac9af89f0a@gmail.com>

stackforge/python-tackerclient,openstack/python-tackerclient,openstack/python-tackerclient,stackforge/python-tackerclient