Need to return a unicode string in the xml factory.

The underlying data is put into json documents that expect unicode
strings not byte strings.  In python2 lxml provided strings that were
auto coerced but in python3 it provides byte strings.

common/lib/capa/capa/tests/response_xml_factory.py

@@ -102,7 +102,7 @@ class ResponseXMLFactory(six.with_metaclass(ABCMeta, object)):
                 explanation_div.set("class", "detailed-solution")
                 explanation_div.text = explanation_text
 
-        return etree.tostring(root)
+        return etree.tostring(root).decode('utf-8')
 
     @staticmethod
     def textline_input_xml(**kwargs):