sagemathgh-39182: Define __iter__ method in iterator types

    
This is required for iterator types and seems to be enforced in Python
3.13

https://docs.python.org/3/library/stdtypes.html#iterator-types
    
URL: sagemath#39182
Reported by: Antonio Rojas
Reviewer(s): user202729

src/sage/matroids/extension.pyx

@@ -211,6 +211,9 @@ cdef class LinearSubclassesIter:
 
         self._nodes = [first_cut]
 
+    def __iter__(self):
+        return self
+
     def __next__(self):
         """
         Return the next linear subclass.

src/sage/matroids/set_system.pyx

@@ -772,6 +772,9 @@ cdef class SetSystemIterator:
         self._pointer = -1
         self._len = len(H)
 
+    def __iter__(self):
+        return self
+
     def __next__(self):
         """
         Return the next subset of a SetSystem.