Adds Event stub to multiprocessing (#1247)

* Adds Event type to multiprocessing

* Add event and context stub to multiprocessing stub.  Updates per recommendations.

* Adding missing newline at EOF.

* Stubbing just BaseContext and not any of the member functions.

* Fix flake8 extra line.

* Fix comment.

* Add todo for BaseContext, make ctx of Any type, and remove multiprocessing.context

stdlib/3/multiprocessing/__init__.pyi

@@ -14,6 +14,14 @@ class AsyncResult():
     def ready(self) -> bool: ...
     def successful(self) -> bool: ...
 
+class Event(object):
+    # TODO: change ctx to BaseContext once a stub exists for multiprocessing.context
+    def __init__(self, *, ctx: Any) -> None: ...
+    def is_set(self) -> bool: ...
+    def set(self) -> None: ...
+    def clear(self) -> None: ...
+    def wait(self, timeout: Optional[int] = None) -> bool: ...
+
 class Pool():
     def __init__(self, processes: Optional[int] = None,
                  initializer: Optional[Callable[..., None]] = None,