title: Prepend or append PI before / after the root element with lxml
body: <p>With lxml, how can I prepend processing instructions before the root element or append PIs after de root element with lxml.</p>
  
  <p>Currently, the following example doesn't work:</p>
  
  <pre class="lang-py prettyprint-override"><code>from lxml import etree
  
  root = etree.XML("&lt;ROOT/&gt;")
  root.addprevious(etree.ProcessingInstruction("foo"))
  print(etree.tounicode(root))
  </code></pre>
  
  <p>I get:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;ROOT/&gt;
  </code></pre>
  
  <p>Instead of:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;?foo?&gt;&lt;ROOT/&gt;
  </code></pre>