Handle result from PyObject_VisitManagedDict

[maxbachmann]

Without this the relationship between __dict__ and self isn't found.

[rwgk]

How much trouble is it to add a test?

If adding a test is too much trouble: could you please explain in the PR description what the problem was, and why this is the correct fix?

I'm totally willing to believe this fix is needed and correct, but without any test or explanation it's very opaque.

[maxbachmann]

This is the same handling as PY_VISIT.

#define Py_VISIT(op)                                                    \
    do {                                                                \
        if (op) {                                                       \
            int vret = visit(_PyObject_CAST(op), arg);                  \
            if (vret)                                                   \
                return vret;                                            \
        }                                                               \
    } while (0)

I had the same bug in the Python wrapper I am implementing at my company and so checked whether others had this problem as well.

For us this fixes the following case:

    instance = m.DynamicAttr()
    instance.a = "test"
    assert instance in gc.get_referrers(instance.__dict__)

For pybind11 this unit test still fails due to python/cpython#130327. It should presumably work once we upgrade to 3.14.4 which was released yesterday.

[maxbachmann]

Worth noting that cpython doesn't always properly handle the result either (python/cpython#148275).