Enforce positional arguments for set_threshold in gc (#12350)

stdlib/gc.pyi

@@ -34,4 +34,4 @@ if sys.version_info >= (3, 9):
 
 def isenabled() -> bool: ...
 def set_debug(flags: int, /) -> None: ...
-def set_threshold(threshold0: int, threshold1: int = ..., threshold2: int = ...) -> None: ...
+def set_threshold(threshold0: int, threshold1: int = ..., threshold2: int = ..., /) -> None: ...