Involve instance attributes in GC

I copied subtype_traverse and subtype_clear from cpython but
didn't update them to support our hcattrs

src/runtime/objmodel.cpp

@@ -827,6 +827,10 @@ static int subtype_traverse(PyObject* self, visitproc visit, void* arg) noexcept
             Py_VISIT(*dictptr);
     }
 
+    if (type->attrs_offset != base->attrs_offset) {
+        Py_TRAVERSE(*self->getHCAttrsPtr());
+    }
+
     if (type->tp_flags & Py_TPFLAGS_HEAPTYPE)
         /* For a heaptype, the instances count as references
            to the type.          Traverse the type so the collector
@@ -861,6 +865,10 @@ static int subtype_clear(PyObject* self) noexcept {
             Py_CLEAR(*dictptr);
     }
 
+    if (type->attrs_offset != base->attrs_offset) {
+        self->getHCAttrsPtr()->clear();
+    }
+
     if (baseclear)
         return baseclear(self);
     return 0;

test/tests/instance_cycles.py

+class C(object):
+    pass
+
+c = C()
+c.t = (c,)

test/tests/set-recursion.py

-# expected: reffail
 class C(object):
     def __init__(self):
         self.t = set([self])

test/tests/tuples.py

-# expected: reffail
 # Some parsing tests:
 print ((1, 2),)
 print (1, 2, 3)