With ET 1.3, you should have an explicit keyword argument "xml_declaration":

# ----
        if xml_declaration or (xml_declaration is None and
                               encoding not in ("utf-8", "us-ascii")):
            if method == "xml":
                write("<?xml version='1.0' encoding='%s'?>\n" % encoding)
# ----

In ET 1.2.6, the same snippet looks like:
# ----
        if encoding != "utf-8" and encoding != "us-ascii":
            file.write("<?xml version='1.0' encoding='%s'?>\n" % encoding)
# ----