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Here's some fairly minimal Python code that produces the signal:
### begin example ###
import os
import time
import _thread
try:
 os.execv('/usr/bin/dorothyq', ['dorothyq'])
except OSError:
 pass
def f():
 time.sleep(1.0) # probably irrelevant to the failure
_thread.start_new(f, ())
### end example ###
It looks as though the failed os.execv call messes something up internally, so that any attempt thereafter to start a thread produces this signal. I can't see anything obviously wrong with the os.execv implementation (see posix_execv in Modules/posixmodule.c).
There's still the question of what changed between 2.x and 3.x: on 2.x, this buildbot seems perfectly happy.