The patch looks good.
However, I've had two failures while testing it:
- a BrokenBarrierError on test_default_timeout: I see you've already increased the timeout from the original threading code, but you can probably double it (we have some slow buildbots, and creating processes is expensive on Windows)
- TestNumberOfObjects is failing too:
"""
test_number_of_objects (test.test_multiprocessing.WithManagerTestZZZNumberOfObjects) ...   b6cb776c:       refcount=1
    <threading.Barrier object at 0xb6cb776c>
  b6d72dfc:       refcount=2
    <multiprocessing.pool.Pool object at 0xb6d72dfc>
  b6cb776c:       refcount=1
    <threading.Barrier object at 0xb6cb776c>
  b6d72dfc:       refcount=2
    <multiprocessing.pool.Pool object at 0xb6d72dfc>
FAIL
"""