gh-115398: Expose Expat >=2.6.0 reparse deferral API (CVE-2023-52425) (GH-115623)

Allow controlling Expat >=2.6.0 reparse deferral (CVE-2023-52425) by adding five new methods:

- `xml.etree.ElementTree.XMLParser.flush`
- `xml.etree.ElementTree.XMLPullParser.flush`
- `xml.parsers.expat.xmlparser.GetReparseDeferralEnabled`
- `xml.parsers.expat.xmlparser.SetReparseDeferralEnabled`
- `xml.sax.expatreader.ExpatParser.flush`

Based on the "flush" idea from https://github.com/python/cpython/pull/115138#issuecomment-1932444270 .

### Notes

- Please treat as a security fix related to CVE-2023-52425.

Includes code suggested-by: Snild Dolkow <snild@sony.com>
and by core dev Serhiy Storchaka.

Lib/test/test_sax.py

@@ -19,6 +19,7 @@ from xml.sax.xmlreader import InputSource, AttributesImpl, AttributesNSImpl
 from io import BytesIO, StringIO
 import codecs
 import os.path
+import pyexpat
 import shutil
 import sys
 from urllib.error import URLError
@@ -1214,6 +1215,56 @@ class ExpatReaderTest(XmlTestBase):
 
         self.assertEqual(result.getvalue(), start + b"<doc>text</doc>")
 
+    def test_flush_reparse_deferral_enabled(self):
+        if pyexpat.version_info < (2, 6, 0):
+            self.skipTest(f'Expat {pyexpat.version_info} does not support reparse deferral')
+
+        result = BytesIO()
+        xmlgen = XMLGenerator(result)
+        parser = create_parser()
+        parser.setContentHandler(xmlgen)
+
+        for chunk in ("<doc", ">"):
+            parser.feed(chunk)
+
+        self.assertEqual(result.getvalue(), start)  # i.e. no elements started
+        self.assertTrue(parser._parser.GetReparseDeferralEnabled())
+
+        parser.flush()
+
+        self.assertTrue(parser._parser.GetReparseDeferralEnabled())
+        self.assertEqual(result.getvalue(), start + b"<doc>")
+
+        parser.feed("</doc>")
+        parser.close()
+
+        self.assertEqual(result.getvalue(), start + b"<doc></doc>")
+
+    def test_flush_reparse_deferral_disabled(self):
+        result = BytesIO()
+        xmlgen = XMLGenerator(result)
+        parser = create_parser()
+        parser.setContentHandler(xmlgen)
+
+        for chunk in ("<doc", ">"):
+            parser.feed(chunk)
+
+        if pyexpat.version_info >= (2, 6, 0):
+            parser._parser.SetReparseDeferralEnabled(False)
+
+        self.assertEqual(result.getvalue(), start)  # i.e. no elements started
+        self.assertFalse(parser._parser.GetReparseDeferralEnabled())
+
+        parser.flush()
+
+        self.assertFalse(parser._parser.GetReparseDeferralEnabled())
+        self.assertEqual(result.getvalue(), start + b"<doc>")
+
+        parser.feed("</doc>")
+        parser.close()
+
+        self.assertEqual(result.getvalue(), start + b"<doc></doc>")
+
     # ===== Locator support
 
     def test_expat_locator_noinfo(self):