Backport to 2.2.x:

Backport to 2.2.x:

Address SF bug 519621: slots weren't traversed by GC.

While I was at it, I added a tp_clear handler and changed the
tp_dealloc handler to use the clear_slots helper for the tp_clear
handler.

Also set mp->flags = READONLY for the __weakref__ pseudo-slot.

[Note that I am *not* backporting the part of that patch that
tightened the __slot__ rules.]