Merged manually from 2.7 branch to 3.x trunk.

Merged manually from 2.7 branch to 3.x trunk.

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  r79925 | nick.coghlan | 2010-04-10 16:24:36 +0200 (sam. 10 avril 2010)

  Try to turn some buildbots green by allowing test_multiprocessing to
  pass even if it hits the sys.exc_clear code in the threading module, and
  improve the test coverage by making the ctypes dependencies a bit more
  granular (two of the cited ctypes objects don't exist on my system)
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