Fix type stubs in locks.pyi (#1250)

stdlib/3.4/asyncio/locks.pyi

@@ -1,8 +1,9 @@
-from typing import Any, Callable, Generator, Iterable, Iterator, List, TypeVar, Union, Optional
+from typing import Any, Callable, Generator, Iterable, Iterator, List, Type, TypeVar, Union, Optional
 
 from .coroutines import coroutine
 from .events import AbstractEventLoop
-from .futures import Future
+from .futures import Future, Awaitable
+from types import TracebackType
 
 _T = TypeVar('_T')
 
@@ -17,18 +18,18 @@ class _ContextManagerMixin(Future[_ContextManager]):
     # Apparently this exists to *prohibit* use as a context manager.
     def __enter__(self) -> object: ...
     def __exit__(self, *args: Any) -> None: ...
-    def __aenter__(self): ...
-    def __aexit__(self, exc_type, exc, tb): ...
+    def __aenter__(self) -> Awaitable[None]: ...
+    def __aexit__(self, exc_type: Optional[Type[BaseException]], exc: Optional[BaseException], tb: Optional[TracebackType]) -> Awaitable[None]: ...
 
 class Lock(_ContextManagerMixin):
-    def __init__(self, *, loop: AbstractEventLoop = None) -> None: ...
+    def __init__(self, *, loop: Optional[AbstractEventLoop] = ...) -> None: ...
     def locked(self) -> bool: ...
     @coroutine
     def acquire(self) -> Generator[Any, None, bool]: ...
     def release(self) -> None: ...
 
 class Event:
-    def __init__(self, *, loop: AbstractEventLoop = None) -> None: ...
+    def __init__(self, *, loop: Optional[AbstractEventLoop] = ...) -> None: ...
     def is_set(self) -> bool: ...
     def set(self) -> None: ...
     def clear(self) -> None: ...
@@ -36,7 +37,7 @@ class Event:
     def wait(self) -> Generator[Any, None, bool]: ...
 
 class Condition(_ContextManagerMixin):
-    def __init__(self, lock: Lock = None, *, loop: AbstractEventLoop = None) -> None: ...
+    def __init__(self, lock: Lock = None, *, loop: Optional[AbstractEventLoop] = ...) -> None: ...
     def locked(self) -> bool: ...
     @coroutine
     def acquire(self) -> Generator[Any, None, bool]: ...