Call PyObject_GC_UnTrack in structseq dealloc (GH-13751)

diff --git a/Objects/structseq.c b/Objects/structseq.c
index 3d857f7..2c25e16 100644
--- a/Objects/structseq.c
+++ b/Objects/structseq.c
@@ -77,6 +77,7 @@
 {
     Py_ssize_t i, size;
     PyTypeObject *tp;
+    PyObject_GC_UnTrack(obj);
 
     tp = (PyTypeObject *) Py_TYPE(obj);
     size = REAL_SIZE(obj);