I've found more unsafe code in Objects/setobject.c.
This code makes Python 3.1.2 segfault by using a bug in function set_merge:

class bad:
	def __eq__(self, other):
		if be_bad:
			set2.clear()
			raise Exception
		return self is other
	def __hash__(self):
		return 0
be_bad = False
set1 = {bad()}
set2 = {bad() for i in range(2000)}
be_bad = True
set1.update(set2)

Function set_symmetric_difference_update has a similar bug.

Another bug in set_symmetric_difference_update:

class bad:
	def __init__(self):
		print("Creating", id(self))
	def __del__(self):
		print("Deleting", id(self))
	def __eq__(self, other):
		print("Comparing", id(self), "and", id(other))
		if be_bad:
			dict2.clear()
		return self is other
	def __hash__(self):
		return 0
be_bad = False
set1 = {bad()}
dict2 = {bad(): None}
be_bad = True
set1.symmetric_difference_update(dict2)