Instead of requiring to import Credentials from the kasa package, we should allow creating instances without it.
@sdb9696 suggested to allow passing the username and the password as kwargs, which sounds like a good idea:

from kasa import Discover

dev = Discover.discover_single("127.0.0.1", username="foo", password="bar")
devs = Discover.discover(target="192.168.0.255", username="foo", password="bar")