Parse XML file python

I have an XML file (made with ONIXEDIT) with doctype :

<!DOCTYPE ONIXMessage SYSTEM "http://www.editeur.org/onix/2.1/reference/onix-international.dtd>

the file contains XHTML entities, when i try to parse using xml.etree.ElementTree, it gives this error :

xml.etree.ElementTree.ParseError: undefined entity  

How can i solve this ?