ronaldoussoren, I have tested (Debian 9) substituting "_PyObject_GC_UNTRACK(m);" to "PyObject_GC_UnTrack(m);", and although the tests pass, I'm can still see the segfault error.

diff --git a/Objects/methodobject.c b/Objects/methodobject.c
index cfea8cf410..cb8e279c7d 100644
--- a/Objects/methodobject.c
+++ b/Objects/methodobject.c
@@ -84,7 +84,7 @@ PyCFunction_GetFlags(PyObject *op)
 static void
 meth_dealloc(PyCFunctionObject *m)
 {
-    _PyObject_GC_UNTRACK(m);
+    PyObject_GC_UnTrack(m);
     if (m->m_weakreflist != NULL) {
         PyObject_ClearWeakRefs((PyObject*) m);
     }