Support for Python 3.12?

[maresb]

Hi! I've been building llist for conda-forge. We are preparing for Python 3.12 and the test suite is failing. It seems like test_cyclic_list_destruction_does_not_release_extra_None_refs ends up in an infinite (or at least unreasonably long) loop:

test_cyclic_list_destruction_does_not_release_extra_None_refs (sllist_test.testsllist.test_cyclic_list_destruction_does_not_release_extra_None_refs) ... gc: collecting generation 0...
gc: objects in each generation: 704 0 11671
gc: objects in permanent generation: 375
gc: done, 693 unreachable, 0 uncollectable, 0.0001s elapsed
gc: collecting generation 0...
gc: objects in each generation: 702 9 11671
gc: objects in permanent generation: 375
gc: done, 699 unreachable, 0 uncollectable, 0.0001s elapsed
gc: collecting generation 0...
gc: objects in each generation: 701 12 11671
gc: objects in permanent generation: 375
gc: done, 696 unreachable, 0 uncollectable, 0.0001s elapsed
gc: collecting generation 1...
gc: objects in each generation: 701 17 11671
gc: objects in permanent generation: 375
gc: done, 708 unreachable, 0 uncollectable, 0.0001s elapsed
gc: collecting generation 0...
gc: objects in each generation: 702 0 11680
gc: objects in permanent generation: 375
gc: done, 699 unreachable, 0 uncollectable, 0.0001s elapsed
gc: collecting generation 0...
gc: objects in each generation: 701 3 11680
gc: objects in permanent generation: 375
gc: done, 696 unreachable, 0 uncollectable, 0.0001s elapsed
gc: collecting generation 0...
...

Any idea what's going on or how to fix it? Unfortunately I'm not so proficient in C so I can't be of much help myself. Thanks!