[Stackless] Exception passing patch

[Peter Ingebretson]

> If your patch doesn't already do so, I'd suggest that
> send_exception can take up to three arguments, with the same
> semantics as "raise"

My patch doesn't currently support that syntax, although it was one of the things I considered.  I had two arguments against supporting an optional third traceback argument:

1) It changes the semantics of send_exception, so it would have the potential to break any codebases depending on the current implementation.  For example, send_exception(MyException, "My Message", my_traceback) would formerly have constructed MyException("My Message", my_traceback) but with this proposal would instead behave as:
    e = MyException("My Message")
    e.__traceback__ = my_traceback
This example is contrived, but I didn't want to potentially break existing code without a strong reason.

2) Python 3k has dropped support for the 'raise E, V, T' syntax in favor of 'raise E(V)' or 'e = E(V); e.__traceback__ = T; raise e'.  My patch is intended for the py3k branch of Stackless, and I didn't want to add a feature which added semantics that had been removed from 'raise' itself.

Thanks,
Peter