GC: Attributes can have values that did not get allocated by the GC

Using the C API it's possible to create a Box instance without using our GC alloc.
For example have a look at md5module.c: PyDict_SetItemString(d, "MD5Type", (PyObject *)&MD5type);

src/runtime/types.cpp

@@ -183,7 +183,7 @@ extern "C" void boxGCHandler(GCVisitor* v, Box* b) {
                 HCAttrs::AttrList* attr_list = attrs->attr_list;
                 assert(attr_list);
                 v->visit(attr_list);
-                v->visitRange((void**)&attr_list->attrs[0], (void**)&attr_list->attrs[nattrs]);
+                v->visitPotentialRange((void**)&attr_list->attrs[0], (void**)&attr_list->attrs[nattrs]);
             }
         }
     } else {

test/tests/hashlib_test.py

+# expected: fail
+# - warnings about PyString_AsString(), since that is allowed to be modified
 import hashlib
 
 #for m in [hashlib.md5(), hashlib.sha1(), hashlib.sha256(), hashlib.sha512()]: