py3: etree.fromstring requires bytes input or XML fragments without declaration

bt5/erp5_core_test/TestTemplateItem/portal_components/test.erp5.testProxyField.py

@@ -333,7 +333,7 @@ return printed
 
     # Test xml serialisation of form.
     xml_string = formToXML(form)
-    xml_tree = etree.fromstring(xml_string)
+    xml_tree = etree.fromstring(xml_string.encode())
     field_node = xml_tree.find('groups/group/fields/field')
     self.assertEqual(field_node.find('type').text, 'ProxyField')
     self.assertTrue(field_node.find('delegated_list/title') is not None)