Test failures under Python 3.13

[mdickinson]

Under Python 3.13.0a6 we get the following test failures:

======================================================================
FAIL: test_analyze_simple_cycle (refcycle.test.test_object_graph.TestObjectGraph.test_analyze_simple_cycle)
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Traceback (most recent call last):
  File "/Users/mdickinson/Repositories/mdickinson/refcycle/src/refcycle/test/test_object_graph.py", line 406, in test_analyze_simple_cycle
    self.assertEqual(len(sccs), 1)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
AssertionError: 3 != 1

======================================================================
FAIL: test_cycles_created_by (refcycle.test.test_refcycle.TestRefcycle.test_cycles_created_by)
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Traceback (most recent call last):
  File "/Users/mdickinson/Repositories/mdickinson/refcycle/src/refcycle/test/test_refcycle.py", line 56, in test_cycles_created_by
    self.assertEqual(len(object_graph), 4)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 2 != 4

These appear to be the result of logic for dematerialization/rematerialization of instance dicts; see for example python/cpython#106539.