[3.13] gh-128302: Fix bugs in xml.dom.xmlbuilder (GH-128284) (#128582)

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 6ea04da27036eaa69d65150148bb8c537d9beacf)

Co-authored-by: Stephen Morton <git@tungol.org>