By replacing temporary immutability with temporary hashability, does 
this approach create the possibility that someone could mutate the key-
set during a search?  Is it possible to get the __eq__ check out-of-
sync with the __hash__ value?

Also, after a quick look at the patch, I'm not too keen on any 
modifications to set_swap_bodies() nor with changing the signature of 
set_contains_key().