bpo-35013: Add more type checks for children of Element. (GH-9944)

It is now guarantied that children of xml.etree.ElementTree.Element
are Elements (at least in C implementation). Previously methods
__setitem__(), __setstate__() and __deepcopy__() could be used for
adding non-Element children.

Lib/test/test_xml_etree.py

@@ -1795,6 +1795,28 @@ class BasicElementTest(ElementTestCase, unittest.TestCase):
         self.assertRaises(TypeError, e.append, 'b')
         self.assertRaises(TypeError, e.extend, [ET.Element('bar'), 'foo'])
         self.assertRaises(TypeError, e.insert, 0, 'foo')
+        e[:] = [ET.Element('bar')]
+        with self.assertRaises(TypeError):
+            e[0] = 'foo'
+        with self.assertRaises(TypeError):
+            e[:] = [ET.Element('bar'), 'foo']
+
+        if hasattr(e, '__setstate__'):
+            state = {
+                'tag': 'tag',
+                '_children': [None],  # non-Element
+                'attrib': 'attr',
+                'tail': 'tail',
+                'text': 'text',
+            }
+            self.assertRaises(TypeError, e.__setstate__, state)
+
+        if hasattr(e, '__deepcopy__'):
+            class E(ET.Element):
+                def __deepcopy__(self, memo):
+                    return None  # non-Element
+            e[:] = [E('bar')]
+            self.assertRaises(TypeError, copy.deepcopy, e)
 
     def test_cyclic_gc(self):
         class Dummy:
@@ -1981,26 +2003,6 @@ class BadElementTest(ElementTestCase, unittest.TestCase):
         elem = b.close()
         self.assertEqual(elem[0].tail, 'ABCDEFGHIJKL')
 
-    def test_element_iter(self):
-        # Issue #27863
-        state = {
-            'tag': 'tag',
-            '_children': [None],  # non-Element
-            'attrib': 'attr',
-            'tail': 'tail',
-            'text': 'text',
-        }
-
-        e = ET.Element('tag')
-        try:
-            e.__setstate__(state)
-        except AttributeError:
-            e.__dict__ = state
-
-        it = e.iter()
-        self.assertIs(next(it), e)
-        self.assertRaises(AttributeError, next, it)
-
     def test_subscr(self):
         # Issue #27863
         class X: