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).

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