Issue 3731: import warning in multiprocessing

Indeed. _multiprocessing.so compiles fine but afterwards I get:

*** WARNING: importing extension "_multiprocessing" failed with <type
'exceptions.AttributeError'>: 'module' object has no attribute 'SemLock'


And if I try manually:

>>> import _multiprocessing
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/antoine/cpython/__svn__/Lib/multiprocessing/__init__.py",
line 148, in <module>
    from multiprocessing.synchronize import (Lock, RLock, Condition, Event,
  File
"/home/antoine/cpython/__svn__/Lib/multiprocessing/synchronize.py", line
29, in <module>
    SEM_VALUE_MAX = _multiprocessing.SemLock.SEM_VALUE_MAX
AttributeError: 'module' object has no attribute 'SemLock'


Removing the .pyc files doesn't help.