Previously, we would just call these "conservative python" objects,
and scan their memory conservatively.  The problem with this is that
the builtin type might have defined a custom GC handler that needs to
be called in addition to the conservative scanning (ie it stores GC
pointers out-of-band that are not discoverable via other gc pointers).

We had dealt with these kinds of issues before which is why I added
the "conservative python kind", but I think the real solution here is
to say that to the GC, these objects are just python objects, and
then let the type machinery decide how to scan the objects, including
how to handle the inheritance rules.  We were already putting
"conservativeGCHandler" as the gc_handler on these conservative types,
so let's use it.