Add missing type hints to atexit.register() (#571)

stdlib/3/atexit.pyi

@@ -5,5 +5,5 @@ from typing import Any, Callable
 def _clear() -> None: ...
 def _ncallbacks() -> int: ...
 def _run_exitfuncs() -> None: ...
-def register(func: Callable[..., Any], *args, **kwargs) -> Callable[..., Any]: ...
+def register(func: Callable[..., Any], *args: Any, **kwargs: Any) -> Callable[..., Any]: ...
 def unregister(func: Callable[..., Any]) -> None: ...