gh-102351: bump MAGIC_NUMBER

Author: iritkatriel

Lib/importlib/_bootstrap_external.py

@@ -434,6 +434,7 @@ def _write_atomic(path, data, mode=0o666):
 #     Python 3.12a5 3518 (Add RETURN_CONST instruction)
 #     Python 3.12a5 3519 (Modify SEND instruction)
 #     Python 3.12a5 3520 (Remove PREP_RERAISE_STAR, add CALL_INTRINSIC_2)
+#     Python 3.12a5 3521 (Fix refleak)
 
 #     Python 3.13 will start with 3550