test_xml_etree.py: Fix for Expat >=2.6.0 with reparse deferral

Fix etree XMLPullParser tests for Expat >=2.6.0 with reparse deferral

Combined with gh#python/cpython!31453
bpo-46811: Make test suite support Expat >=2.4.5 (pythonGH-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
(cherry picked from commit 2cae938)

Co-authored-by: Sebastian Pipping <[email protected]>
Fixes: gh#python#115133
From-PR: gh#python/cpython!115138
Patch: CVE-2023-52425-libexpat-2.6.0-backport-15.6.patch

Lib/test/test_xml_etree.py

@@ -1046,7 +1046,11 @@ def assert_event_tags(self, parser, expected):
                          expected)
 
     def test_simple_xml(self):
-        for chunk_size in (None, 1, 5):
+        # if is_expat_2_6_0:
+        chunks = (None,)
+        # else:
+        #     chunks = (None, 1, 5)
+        for chunk_size in chunks:
             with self.subTest(chunk_size=chunk_size):
                 expected_events = []
                 parser = ET.XMLPullParser()