lint: fix typing

Author: busunkim96

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/base.py.j2

@@ -30,8 +30,8 @@ class {{ service.name }}Transport(abc.ABC):
             self, *,
             host: str{% if service.host %} = '{{ service.host }}'{% endif %},
             credentials: credentials.Credentials = None,
-            credentials_file: str = None,
-            scopes: Optional[typing.Sequence[str]] = AUTH_SCOPES,
+            credentials_file: typing.Optional[str] = None,
+            scopes: typing.Optional[typing.Sequence[str]] = AUTH_SCOPES,
             **kwargs,       
             ) -> None:
         """Instantiate the transport.