Fix asyncio.__all__: export also unix_events and windows_events symbols

For example, on Windows, it was not possible to get ProactorEventLoop or
DefaultEventLoopPolicy using "from asyncio import *".

Lib/asyncio/__init__.py

@@ -29,12 +29,6 @@ from .subprocess import *
 from .tasks import *
 from .transports import *
 
-if sys.platform == 'win32':  # pragma: no cover
-    from .windows_events import *
-else:
-    from .unix_events import *  # pragma: no cover
-
-
 __all__ = (coroutines.__all__ +
            events.__all__ +
            futures.__all__ +
@@ -45,3 +39,10 @@ __all__ = (coroutines.__all__ +
            subprocess.__all__ +
            tasks.__all__ +
            transports.__all__)
+
+if sys.platform == 'win32':  # pragma: no cover
+    from .windows_events import *
+    __all__ += windows_events.__all__
+else:
+    from .unix_events import *  # pragma: no cover
+    __all__ += unix_events.__all__