py: Replace mp_const_stop_iteration object with MP_OBJ_NULL.

tests/basics/iter2.py

@@ -0,0 +1,23 @@
+# user defined iterator used in something other than a for loop
+
+class MyStopIteration(StopIteration):
+    pass
+
+class myiter:
+    def __init__(self, i):
+        self.i = i
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        if self.i == 0:
+            raise StopIteration
+        elif self.i == 1:
+            raise StopIteration(1)
+        elif self.i == 2:
+            raise MyStopIteration
+
+print(list(myiter(0)))
+print(list(myiter(1)))
+print(list(myiter(2)))