title: Argument must be bytes or unicode, got '_Element'
body: <p>I am using Python 3.3 and running into this error</p>
  
  <pre><code>from lxml import etree
  xmlns = "http://www.fpml.org/FpML-5/confirmation"
  xsi  = "http://www.w3.org/2001/XMLSchema-instance"
  fpmlVersion="http://www.fpml.org/FpML-5/confirmation ../../fpml-main-5-6.xsd http://www.w3.org/2000/09/xmldsig# ../../xmldsig-core-schema.xsd"
  page = etree.Element("{"+xmlns+"}dataDocument",nsmap={None:xmlns,'xsi':xsi })
  page.set("fpmlVersion", fpmlVersion)
  doc = etree.SubElement(page,trade)
  s = etree.tostring(doc, xml_declaration=True,encoding="UTF-8",pretty_print=True)
  print (s)
  </code></pre>
  
  
  
  <pre><code>TypeError: Argument must be bytes or unicode, got '_Element'
  </code></pre>
  
  <p>I want the output to be</p>
  
  <pre><code>&lt;dataDocument xmlns="http://www.fpml.org/FpML-5/confirmation" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  fpmlVersion="http://www.tradfpml.org/FpML-5/confirmation ../../fpml-main-5-6.xsd http://www.w3.org/2000/09/xmldsig# ../../xmldsig-core-schema.xsd"&gt;
  &lt;trade&gt;
  &lt;/trade&gt;
  &lt;/dataDocument&gt;
  </code></pre>