Merged revisions 87740 via svnmerge from

Merged revisions 87740 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/branches/py3k

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  r87740 | gregory.p.smith | 2011-01-04 10:33:38 -0800 (Tue, 04 Jan 2011) | 6 lines

  Fix the new bug introduced in the r87710 fix for issue 6643.  DummyThread
  deletes its _block attribute, deal with that.  This prevents an uncaught
  exception in a thread during test_thread.

  This refactors a bit to better match what I did in the r87727 backport to 2.7.
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