input: Write a function to construct an XML document using Python which includes a list of items as sub-elements under a root element named 'data'. The list of items is provided as an input to the function. The function should output the XML document to a file named 'data.xml'.
  
  def construct_xml(items):
      import xml.etree.ElementTree as ET
  
      # creating the root element
      data = ET.Element('data')
  
      # loop over items
      for item in items:
          # creating sub elements
          item_element = ET.SubElement(data, 'item')
  
          # assign the value for the sub elements
          item_element.text = item
  
      # write the XML Tree
      tree = ET.ElementTree(data)
      tree.write('data.xml')
  
  assert construct_xml(["Item1", "Item2", "Item3"])
output: def construct_xml(items):
      import xml.etree.ElementTree as ET
  
      data = ET.Element('data')
  
      for item in items:
          item_element = ET.SubElement(data, 'item')
          item_element.text = item
  
      tree = ET.ElementTree(data)
      tree.write('data.xml')