title: lxml not supports multiple attributes filter on xpath
body: <p>I want to select nodes with multiple attributes using xpath</p>
  
  <p>is there any workaround for my problem ? </p>
  
  <p><code>node.xpath('.//td[@colspan=6 @class="mbw"]')</code></p>
  
  <pre><code>  File "lxml.etree.pyx", line 1507, in lxml.etree._Element.xpath (src/lxml/lxml.etree.c:52198)
    File "xpath.pxi", line 307, in lxml.etree.XPathElementEvaluator.__call__ (src/lxml/lxml.etree.c:152124)
    File "xpath.pxi", line 227, in lxml.etree._XPathEvaluatorBase._handle_result (src/lxml/lxml.etree.c:151097)
    File "xpath.pxi", line 212, in lxml.etree._XPathEvaluatorBase._raise_eval_error (src/lxml/lxml.etree.c:150896)
  lxml.etree.XPathEvalError: Invalid predicate
  </code></pre>