title: Writing lxml.etree with double quotes header attributes
body: <p>I have created a basic xml tree using <a href="http://lxml.de/tutorial.html" rel="noreferrer">lxml tutorial</a>:</p>
  
  <pre><code>from lxml import etree
  root = etree.Element("root")
  root.append( etree.Element("child1") )
  child2 = etree.SubElement(root, "child2")
  child3 = etree.SubElement(root, "child3")
  print(etree.tostring(root, pretty_print=True, encoding="UTF-8", xml_declaration=True))
  </code></pre>
  
  <p>This produces the following:</p>
  
  <pre><code>&lt;?xml version='1.0' encoding='UTF-8'?&gt;
  &lt;root&gt;
    &lt;child1/&gt;
    &lt;child2/&gt;
    &lt;child3/&gt;
  &lt;/root&gt;
  </code></pre>
  
  <p>My question is, how to produce xml file with double quoted file header, i.e.</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  ....
  </code></pre>