gh-114737: Revert change to ElementTree.iterparse "root" attribute (GH-114755)

Prior to gh-114269, the iterator returned by ElementTree.iterparse was
initialized with the root attribute as None. This restores the previous
behavior.

Lib/test/test_xml_etree.py

@@ -536,7 +536,9 @@ class ElementTreeTest(unittest.TestCase):
         iterparse = ET.iterparse
 
         context = iterparse(SIMPLE_XMLFILE)
+        self.assertIsNone(context.root)
         action, elem = next(context)
+        self.assertIsNone(context.root)
         self.assertEqual((action, elem.tag), ('end', 'element'))
         self.assertEqual([(action, elem.tag) for action, elem in context], [
                 ('end', 'element'),

Lib/xml/etree/ElementTree.py

@@ -1256,8 +1256,8 @@ def iterparse(source, events=None, parser=None):
                 source.close()
 
     it = IterParseIterator()
+    it.root = None
     wr = weakref.ref(it)
-    del IterParseIterator
     return it