gh-159739: Collect grapheme iterator and segment type cycles

Backport to 3.15.

Author: jelle-openai

Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.0c4ae9.rst

@@ -0,0 +1,2 @@
+Fix reference cycles involving the types of grapheme iterators and segments
+created by :func:`unicodedata.iter_graphemes`.

Modules/unicodedata.c

@@ -1989,6 +1989,7 @@ Segment_dealloc(PyObject *self)
 static int
 Segment_traverse(PyObject *self, visitproc visit, void *arg)
 {
+    Py_VISIT(Py_TYPE(self));
     Py_VISIT(((SegmentObject *)self)->string);
     return 0;
 }
@@ -2057,6 +2058,7 @@ GBI_dealloc(PyObject *self)
 static int
 GBI_traverse(PyObject *self, visitproc visit, void *arg)
 {
+    Py_VISIT(Py_TYPE(self));
     Py_VISIT(((GraphemeBreakIterator *)self)->iter.str);
     return 0;
 }