I suspect the problem is caused by nose's isolate plugin.

With this enabled, a copy of sys.modules is saved before each test and then restored after the test.  This causes garbage collection of newly imported modules.  The destructor for the module type causes all globals to be replaced by None.

This will break the atexit function registered by multiprocessing since it depends on globals.

PS. A simple work-around (which does not require people to upgrade to a bugfixed version of Python) is to put

    try:
        import multiprocessing
    except ImportError:
        pass

near the beginning of setup.py.  After this change I don't get the error when running "python setup.py test".