Add asyncio.base_events.__all__ (#7264)

stdlib/asyncio/base_events.pyi

@@ -14,6 +14,10 @@ from typing_extensions import Literal
 if sys.version_info >= (3, 7):
     from contextvars import Context
 
+    __all__ = ("BaseEventLoop",)
+else:
+    __all__ = ["BaseEventLoop"]
+
 _T = TypeVar("_T")
 _ProtocolT = TypeVar("_ProtocolT", bound=BaseProtocol)
 _Context = dict[str, Any]