Fix memory leak: clear managed dict in pybind11_object_dealloc on Python 3.13+ (#5999)

* fix: clear managed dict in pybind11_object_dealloc on Python 3.13+

On Python 3.14, PyObject_GC_Del (tp_free) no longer implicitly clears
the managed dict of objects with Py_TPFLAGS_MANAGED_DICT. Without an
explicit PyObject_ClearManagedDict() call before tp_free(), objects
stored in the __dict__ of py::dynamic_attr() instances have their
refcounts permanently abandoned, causing memory leaks — capsule
destructors for numpy arrays (and other objects) never run.

Adds a regression test: stores a py::capsule in the __dict__ of a
DynamicClass instance and asserts the capsule destructor is called
when the instance is deleted.

* [tests]: mark test_dynamic_attr_dealloc_frees_dict_contents to be strict=False xfail on PYPY

* [docs]: clarify Python version comments in pybind11_object_dealloc

Distinguish between when the API is available (3.13+, where
PyObject_ClearManagedDict was introduced) and when the leak actually
manifests (3.14+, where tp_free stopped implicitly clearing the
managed dict).

---------

Co-authored-by: Yury Matveev <yury.matveev@desy.de>

include/pybind11/detail/class.h

@@ -503,6 +503,17 @@ extern "C" inline void pybind11_object_dealloc(PyObject *self) {
         PyObject_GC_UnTrack(self);
     }
 
+#if PY_VERSION_HEX >= 0x030D0000
+    // PyObject_ClearManagedDict() is available from Python 3.13+. It must be
+    // called before tp_free() because on Python 3.14+ tp_free no longer
+    // implicitly clears the managed dict, which would abandon the refcounts of
+    // objects stored in __dict__ of py::dynamic_attr() types, causing permanent
+    // memory leaks.
+    if (PyType_HasFeature(type, Py_TPFLAGS_MANAGED_DICT)) {
+        PyObject_ClearManagedDict(self);
+    }
+#endif
+
     clear_instance(self);
 
     type->tp_free(self);