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.