import extra helper in retry_unary_async

Author: daniel-sanche

google/api_core/retry/retry_unary_async.py

@@ -73,6 +73,7 @@ async def check_if_exists():
 
 # for backwards compatibility, expose helpers in this module
 from google.api_core.retry.retry_base import if_exception_type  # noqa
+from google.api_core.retry.retry_base import if_transient_error  # noqa
 
 if TYPE_CHECKING:
     import sys