gh-128302: Fix bugs in xml.dom.xmlbuilder (GH-128284)
* Allow DOMParser.parse() to correctly handle DOMInputSource instances
that only have a systemId attribute set.
* Fix DOMEntityResolver.resolveEntity(), which was broken by the
Python 3.0 transition.
* Add Lib/test/test_xml_dom_xmlbuilder.py with few tests.
(cherry picked from commit 6ea04da270)
Co-authored-by: Stephen Morton <git@tungol.org>
Miss Islington (bot)