bpo-46811: Make test suite support Expat >=2.4.5 (GH-31453)

Curly brackets were never allowed in namespace URIs
according to RFC 3986, and so-called namespace-validating
XML parsers have the right to reject them a invalid URIs.

libexpat >=2.4.5 has become strcter in that regard due to
related security issues; with ET.XML instantiating a
namespace-aware parser under the hood, this test has no
future in CPython.

References:
- https://datatracker.ietf.org/doc/html/rfc3968
- https://www.w3.org/TR/xml-names/

Also, test_minidom.py: Support Expat >=2.4.5