[3.10] gh-94207: Fix struct module leak (GH-94239) (GH-94266)

* gh-94207: Fix struct module leak (GH-94239)

Make _struct.Struct a GC type

This fixes a memory leak in the _struct module, where as soon
as a Struct object is stored in the cache, there's a cycle from
the _struct module to the cache to Struct objects to the Struct
type back to the module. If _struct.Struct is not gc-tracked, that
cycle is never collected.

This PR makes _struct.Struct GC-tracked, and adds a regression test.
(cherry picked from commit 6b865349aa)

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>

Lib/test/test_struct.py

@@ -1,12 +1,15 @@
 from collections import abc
 import array
+import gc
 import math
 import operator
 import unittest
 import struct
 import sys
+import weakref
 
 from test import support
+from test.support import import_helper
 from test.support.script_helper import assert_python_ok
 
 ISBIGENDIAN = sys.byteorder == "big"
@@ -671,6 +674,21 @@ class StructTest(unittest.TestCase):
         self.assertIn(b"Exception ignored in:", stderr)
         self.assertIn(b"C.__del__", stderr)
 
+    def test__struct_reference_cycle_cleaned_up(self):
+        # Regression test for python/cpython#94207.
+
+        # When we create a new struct module, trigger use of its cache,
+        # and then delete it ...
+        _struct_module = import_helper.import_fresh_module("_struct")
+        module_ref = weakref.ref(_struct_module)
+        _struct_module.calcsize("b")
+        del _struct_module
+
+        # Then the module should have been garbage collected.
+        gc.collect()
+        self.assertIsNone(
+            module_ref(), "_struct module was not garbage collected")
+
     def test_issue35714(self):
         # Embedded null characters should not be allowed in format strings.
         for s in '\0', '2\0i', b'\0':