title: lxml.etree.XML ValueError for Unicode string
body: <p>I'm transforming <a href="https://gist.github.com/guinslym/5ce47460a31fe4c4046b#file-original_xml-xml" rel="noreferrer">an xml</a> document with <a href="https://gist.github.com/guinslym/5ce47460a31fe4c4046b#file-test_xslt-xslt" rel="noreferrer">xslt</a>. While doing it with python3 I had this following error. But I don't have any errors with python2</p>
  
  <pre><code>-&gt; % python3 cstm/artefact.py
  Traceback (most recent call last):
    File "cstm/artefact.py", line 98, in &lt;module&gt;
      simplify_this_dataset('fisheries-service-des-peches.xml')
    File "cstm/artefact.py", line 85, in simplify_this_dataset
      xslt_root = etree.XML(xslt_content)
    File "lxml.etree.pyx", line 3012, in lxml.etree.XML (src/lxml/lxml.etree.c:67861)
    File "parser.pxi", line 1780, in lxml.etree._parseMemoryDocument (src/lxml/lxml.etree.c:102420)
  ValueError: Unicode strings with encoding declaration are not supported. Please use bytes input or XML fragments without declaration.
  
  #!/usr/bin/env python3
  # vim:fileencoding=UTF-8:ts=4:sw=4:sta:et:sts=4:ai
  # -*- coding: utf-8 -*-
  
  from lxml import etree
  
  def simplify_this_dataset(dataset):
      """Create A simplify version of an xml file
      it will remove all the attributes and assign them as Elements instead
      """
      module_path = os.path.dirname(os.path.abspath(__file__))
      data = open(module_path+'/data/ex-fire.xslt')
      xslt_content = data.read()
      xslt_root = etree.XML(xslt_content)
      dom = etree.parse(module_path+'/../CanSTM_dataset/'+dataset)
      transform = etree.XSLT(xslt_root)
      result = transform(dom)
      f = open(module_path+ '/../CanSTM_dataset/otra.xml', 'w')
      f.write(str(result))
      f.close()
  </code></pre>