Update base.py.j2

Fight with mypy

Author: software-dov

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

@@ -31,7 +31,7 @@ class {{ service.name }}Transport(metaclass=abc.ABCMeta):
             host: str{% if service.host %} = '{{ service.host }}'{% endif %},
             credentials: credentials.Credentials = None,
             credentials_file: str = None,
-            scopes: typing.Sequence[str] = AUTH_SCOPES,
+            scopes: Optional[typing.Sequence[str]] = AUTH_SCOPES,
             ) -> None:
         """Instantiate the transport.