Add a namespace for AttributeValue xsi:type value

This fixes the issue when integrating with onelogin's python3-saml
client:
Element '{urn:oasis:names:tc:SAML:2.0:assertion}AttributeValue',
attribute '{http://www.w3.org/2001/XMLSchema-instance}type': The QName
value 'xs:string' has no corresponding namespace declaration in
scope.

Author: ashimaathri

src/saml2/saml.py

@@ -166,7 +166,9 @@ def set_type(self, typ):
 
         try:
             self.extension_attributes[XSI_TYPE] = typ
+            self.extension_attributes['xmlns:xs'] = XS_NAMESPACE
         except AttributeError:
+            self._extatt['xmlns:xs'] = XS_NAMESPACE
             self._extatt[XSI_TYPE] = typ
 
     def get_type(self):