Handle result from PyObject_VisitManagedDict (#6032)

* Handle result from PyObject_VisitManagedDict

* add unit test

* style: pre-commit fixes

* use different variable name

This avoids a warning on msvc about Py_Visit shadowing the vret variable.

* skip test_get_referrers on unsupported runtimes

The managed-dict referrer check is only known to work on CPython 3.13.13+ and 3.14.4+, while earlier releases and non-CPython interpreters can report different traversal behavior.

Made-with: Cursor

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

tests/test_class.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import gc
 import sys
 from unittest import mock
 
@@ -18,6 +19,13 @@ def refcount_immortal(ob: object) -> int:
     return sys.getrefcount(ob)
 
 
+MANAGED_DICT_GET_REFERRERS_SUPPORTED = (
+    env.CPYTHON
+    and sys.version_info >= (3, 13, 13)
+    and (sys.version_info < (3, 14) or sys.version_info >= (3, 14, 4))
+)
+
+
 def test_obj_class_name():
     expected_name = "UserType" if env.PYPY else "pybind11_tests.UserType"
     assert m.obj_class_name(UserType(1)) == expected_name
@@ -45,6 +53,16 @@ def test_instance(msg):
     assert cstats.alive() == 0
 
 
+@pytest.mark.skipif(
+    not MANAGED_DICT_GET_REFERRERS_SUPPORTED,
+    reason="Requires CPython 3.13.13+ or 3.14.4+ managed dict traversal support",
+)
+def test_get_referrers():
+    instance = m.DynamicAttr()
+    instance.a = "test"
+    assert instance in gc.get_referrers(instance.__dict__)
+
+
 def test_instance_new():
     instance = m.NoConstructorNew()  # .__new__(m.NoConstructor.__class__)