Another possible solution is to explicitly set an exception's __supress_context__ attribute to False (right now impossible because it's the default value). 
If a user can 'turn on' the flag when attaching a different exception (raise X from Y), why not allow 'turning it off'? (symmetry anyone?) right now it is set to False by default and changed to true when 'raising from'.
I suggest changing the default to None, allowing the user to explicitly say: I'm no longer in the previous exception's context.

Feels a bit like solving our hack with another hack (:

And about the PSF contrib agreement, I'll do it as soon as I'm near a printer. too bad we're using pens and not RSA private keys for signatures (-:

   thanks, Alon