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.