[3.12] gh-139400: Make sure that parent parsers outlive their subparsers in pyexpat (GH-139403) (GH-139609)

Within libexpat, a parser created via `XML_ExternalEntityParserCreate`
is relying on its parent parser throughout its entire lifetime.
Prior to this fix, is was possible for the parent parser to be
garbage-collected too early.

(cherry picked from commit 6edb2ddb5f)

Lib/test/test_pyexpat.py

@@ -758,6 +758,42 @@ class ForeignDTDTests(unittest.TestCase):
         self.assertEqual(handler_call_args, [("bar", "baz")])
 
 
+class ParentParserLifetimeTest(unittest.TestCase):
+    """
+    Subparsers make use of their parent XML_Parser inside of Expat.
+    As a result, parent parsers need to outlive subparsers.
+
+    See https://github.com/python/cpython/issues/139400.
+    """
+
+    def test_parent_parser_outlives_its_subparsers__single(self):
+        parser = expat.ParserCreate()
+        subparser = parser.ExternalEntityParserCreate(None)
+
+        # Now try to cause garbage collection of the parent parser
+        # while it's still being referenced by a related subparser.
+        del parser
+
+    def test_parent_parser_outlives_its_subparsers__multiple(self):
+        parser = expat.ParserCreate()
+        subparser_one = parser.ExternalEntityParserCreate(None)
+        subparser_two = parser.ExternalEntityParserCreate(None)
+
+        # Now try to cause garbage collection of the parent parser
+        # while it's still being referenced by a related subparser.
+        del parser
+
+    def test_parent_parser_outlives_its_subparsers__chain(self):
+        parser = expat.ParserCreate()
+        subparser = parser.ExternalEntityParserCreate(None)
+        subsubparser = subparser.ExternalEntityParserCreate(None)
+
+        # Now try to cause garbage collection of the parent parsers
+        # while they are still being referenced by a related subparser.
+        del parser
+        del subparser
+
+
 class ReparseDeferralTest(unittest.TestCase):
     def test_getter_setter_round_trip(self):
         parser = expat.ParserCreate()