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

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 tightened the rules for slot names: they must now be proper
identifiers (ignoring the dirty little fact that <ctype.h> is locale
sensitive).

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

Most of this is a 2.2 bugfix candidate; I'll apply it there myself.