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This seems to be a minimal program to reproduce the problem:
import sys
import os
import platform
platform.mac_ver()
if sys.version_info[0] == 2:
 import urllib
else:
 import urllib.request
if os.fork() == 0:
 print ("about to call getproxies !")
 if sys.version_info[0] == 2:
 print (urllib.getproxies())
 else:
 print (urllib.request.getproxies())
 print ("getproxies has completed !")
else:
 os.wait()
The call to platform.mac_ver is essential, without that call the problem goed away.
This explains why my 2.7 build doesn't crash: I've committed a patch at the EuroPython sprints that replaces the platform.mac_ver implementation by one that reads the OSX version from an XML file instead of calling low-level Apple APIs, and those lowlevel APIs somehow cause the breakage (see also issue #7895)
This means that the problem should go away in the next release for all branches.
mike: are you able to test if the problem goes away when you build python from source using a subversion checkout?
Note that this doesn't mean that the underlying problem is gone, you'll probably still run into problems when the main program is a GUI program that using multiprocessing to start work in the background.