feat: use self-signed jwt for service account (#665)

diff --git a/tests/test__default.py b/tests/test__default.py
index 2738e22..74511f9 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -471,7 +471,7 @@
 
     assert credentials == with_scopes.return_value
     assert project_id == mock.sentinel.project_id
-    with_scopes.assert_called_once_with(MOCK_CREDENTIALS, scopes)
+    with_scopes.assert_called_once_with(MOCK_CREDENTIALS, scopes, default_scopes=None)
 
 
 @mock.patch(