Comment 1 for bug 267825

There isn't currently a way to work around such a broken document.
libxml2 follows the XML spec strictly in that it rejects references to
undeclared entities in the absence of a DTD.

ElementTree lacks DTD support and instead allows you to specify entities
through a parser local "entity" dictionary. lxml could potentially support
a similar interface by intercepting the entity reference resolving at the
SAX layer ("getEntity()" callback function).