Use PyObject_GC_UnTrack in lieu of the old variant

The _PyObject_GC_UNTRACK[1] macro got deprecated in 3.6 and finally
removed in 3.8. Therefore use PyObject_GC_UnTrack instead.

[1] https://docs.python.org/3/c-api/gcsupport.html#c._PyObject_GC_UNTRACK
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