title: Validation against DTD with lxml in python is disturbed by xmlns
body: <p>I'm trying to validate an XML document in Python using lxml.</p>
  
  <p>DTD validation will treat the presence of xmlns namespaces as errors.</p>
  
  <p>This example script</p>
  
  <pre><code>from lxml import etree
  from StringIO import StringIO
  
  dtd = etree.DTD(StringIO("&lt;!ELEMENT a EMPTY&gt;"))
  root = etree.XML("&lt;a&gt;&lt;/a&gt;")
  print(dtd.validate(root))
  root = etree.XML('&lt;a xmlns:sru="http://www.loc.gov/zing/srw/"&gt;&lt;/a&gt;')
  print(dtd.validate(root))
  print dtd.error_log
  </code></pre>
  
  <p>yields</p>
  
  <pre><code>True
  False
  &lt;string&gt;:1:0:ERROR:VALID:DTD_UNKNOWN_ATTRIBUTE: No declaration for attribute xmlns:sru of element a
  </code></pre>
  
  <p>How can I have lxml validate my document against a DTD without complaining about xml namespace?</p>