* fix: Use PyObject_VisitManagedDict() of Python 3.13

Use PyObject_VisitManagedDict() and PyObject_ClearManagedDict() in
pybind11_traverse() and pybind11_clear() on Python 3.13 and newer.

* Add Python 3.13 CI

* tests: don't get numpy/scipy on 3.13 yet

* ci: move 3.13 to upstream

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

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Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>