fix: use grant type password with client secret

When both user credentials and client credentials are supplied, this
library used the grant_type=client_credentials. This fix will instead
retrieve the token with grant_type=password, similar to the Keycloak
admin CLI kcadm.sh.

src/keycloak/openid_connection.py

@@ -311,10 +311,10 @@ def get_token(self):
         The admin token is then set in the `token` attribute.
         """
         grant_type = []
-        if self.client_secret_key:
-            grant_type.append("client_credentials")
-        elif self.username and self.password:
+        if self.username and self.password:
             grant_type.append("password")
+        elif self.client_secret_key:
+            grant_type.append("client_credentials")
 
         if grant_type:
             self.token = self.keycloak_openid.token(