Backport gvanrossum's checkin of revision 1.20:

Thread.__bootstrap(): ignore exceptions in the self.__delete() call in
the finally clause.  An exception here could happen when a daemon
thread exits after the threading module has already been trashed by
the import finalization, and there's not much of a point in trying to
insist doing the cleanup in that stage.

This should fix SF bug ##497111: active_limbo_lock error at program
exit.