config: use keyring.get_password() as a workaround for a kwallet bug

The current implementation of keyring KWallet backend contains a
regression:
get_credential() returns the password as as string.

jaraco/keyring@0b7b41f

openconnect_sso/config.py

@@ -99,7 +99,7 @@ class Credentials(ConfigNode):
 
     @property
     def password(self):
-        return keyring.get_credential(APP_NAME, self.username).password
+        return keyring.get_password(APP_NAME, self.username)
 
     @password.setter
     def password(self, value):