Add Python 3.9 to supported runtimes
Add Python 3.9 to supported runtimes

... because it has been part of tested runtimes since Yoga[1].

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

Change-Id: Ic0c10d26a8e7239daf19e2270659659db125fe4a

openstack/aodh,openstack/aodh