gc: Add typing for gc.is_finalized() (#4589)

stdlib/3/gc.pyi

@@ -33,6 +33,10 @@ def get_referrers(*objs: Any) -> List[Any]: ...
 def get_stats() -> List[Dict[str, Any]]: ...
 def get_threshold() -> Tuple[int, int, int]: ...
 def is_tracked(__obj: Any) -> bool: ...
+
+if sys.version_info >= (3, 9):
+    def is_finalized(__obj: Any) -> bool: ...
+
 def isenabled() -> bool: ...
 def set_debug(__flags: int) -> None: ...
 def set_threshold(threshold0: int, threshold1: int = ..., threshold2: int = ...) -> None: ...