add an asynchronous exception class

[Fredrik Lundh]

Paul Rubin wrote:

> I'd like to suggest adding a builtin abstract class to Python called
> AsynchronousException, which would be a subclass of Exception.  The
> only asynchronous exception I can think of right now is
> KeyboardInterrupt, so KeyboardInterrupt would become a subclass of
> AsynchronousException instead of being a direct subclass of Exception.

PEP 348 addresses this by moving special exceptions out of the
Exception hierarchy:

    http://www.python.org/peps/pep-0348.html

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