input: Write a function to construct an XML document using Python from a list of items. The function should create a root element named 'data' and append each item as a sub-element named 'item' under the root. The function should return an XML element tree that can be written to an XML file. The input list of items will be passed as an argument to the function.
  
  assert construct_xml_document(['Item1', 'Item2', 'Item3']).write('data.xml')
  assert construct_xml_document([]).write('data.xml')
  assert construct_xml_document(['SingleItem']).write('data.xml')
output: def construct_xml_document(items):
      from xml.etree.ElementTree import Element, SubElement, ElementTree
      root = Element('data')
      for item in items:
          SubElement(root, 'item').text = item
      return ElementTree(root)