Previously it was UNTRACKED and the object's gcHandler would scan
it if necessary.  This worked ok most of the time but caused issues
when we subclass from an extension class: the memory for those classes
are allocated conservatively, which meant that a gc handler wasn't
called.

One potential approach would be to allocate extension objects as PYTHON
allocations but with a gcHandler that both scans conservatively and also
knows about the attrs array.  But for now it seems easier to allocate the
attrs array as a PRECISE array (which means it gets scanned) rather than
UNTRACKED, since it will automatically get picked up (and provide better
gc safety in general and remove one of the odd gc cases).

I didn't realize that we weren't using PRECISE at all and that it wasn't
completely working, so this change also required setting kind_data for
PRECISE allocations, and fixing the way we keep this updated during
gc_realloc.