fix mypy

Author: shuoweil

packages/bigframes/bigframes/core/events.py

@@ -26,13 +26,11 @@
 
 import bigframes.session.executor
 
-_DEFAULT = "default"
+_DEFAULT: Literal["default"] = "default"
 
 
 class Subscriber:
-    def __init__(
-        self, callback: Callable[[Event], None], *, publisher: Publisher
-    ):  # noqa: E501
+    def __init__(self, callback: Callable[[Event], None], *, publisher: Publisher):  # noqa: E501
         self._publisher = publisher
         self._callback = callback
         self._subscriber_id = uuid.uuid4()