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

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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>