[GR-68498] Fix the magic-iter.py benchmark

* The previous version led to a lot of polymorphism.
* The C variant in c-magic-iter.py stores `pos` in the instance so we should too.

Author: eregon

graalpython/com.oracle.graal.python.benchmarks/python/micro/magic-iter.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2018, 2024, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2018, 2025, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # The Universal Permissive License (UPL), Version 1.0
@@ -38,8 +38,8 @@
 # SOFTWARE.
 
 class CustomIterator:
-    pos = 0
     def __init__(self, obj):
+        self.pos = 0
         self.__obj = obj
 
     def __iter__(self):