Add Python 3.8 to supported runtimes

Python 3.8 was introduced to tested runtimes in OpenStack projects
during Victoria[1], and a unit test job on Python 3.8 was added.

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

Change-Id: I6e2079619aa306aa34ba74adc491f3b5521b2aa4

Author: kajinamit

setup.cfg

@@ -16,6 +16,7 @@ classifier =
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
 
 [files]
 packages =