feat: add access token credentials (#476)

feat: add access token credentials
diff --git a/google/auth/_default.py b/google/auth/_default.py
index 32e81ba..d7110a1 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -106,10 +106,10 @@
     credential_type = info.get("type")
 
     if credential_type == _AUTHORIZED_USER_TYPE:
-        from google.auth import _cloud_sdk
+        from google.oauth2 import credentials
 
         try:
-            credentials = _cloud_sdk.load_authorized_user_credentials(info)
+            credentials = credentials.Credentials.from_authorized_user_info(info)
         except ValueError as caught_exc:
             msg = "Failed to load authorized user credentials from {}".format(filename)
             new_exc = exceptions.DefaultCredentialsError(msg, caught_exc)