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.
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.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator