remove object fallback for Literal on py3.7

Author: graingert

distributed/actor.py

@@ -28,8 +28,6 @@
     from typing import Literal
 elif TYPE_CHECKING:
     from typing_extensions import Literal
-else:
-    Literal = object
 
 if sys.version_info >= (3, 10):
     from asyncio import Event as _LateLoopEvent