Make loop optional in asyncio.Queue (#2923)

Default value is `None`, so `loop` should be optional.

stdlib/3/asyncio/queues.pyi

@@ -2,7 +2,7 @@ import sys
 from asyncio.events import AbstractEventLoop
 from .coroutines import coroutine
 from .futures import Future
-from typing import Any, Generator, Generic, List, TypeVar
+from typing import Any, Generator, Generic, List, TypeVar, Optional
 
 __all__: List[str]
 
@@ -13,7 +13,7 @@ class QueueFull(Exception): ...
 _T = TypeVar('_T')
 
 class Queue(Generic[_T]):
-    def __init__(self, maxsize: int = ..., *, loop: AbstractEventLoop = ...) -> None: ...
+    def __init__(self, maxsize: int = ..., *, loop: Optional[AbstractEventLoop] = ...) -> None: ...
     def _init(self, maxsize: int) -> None: ...
     def _get(self) -> _T: ...
     def _put(self, item: _T) -> None: ...