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OK, here's a new test_sys.py

> test_sys.py:
>
> - I agree that it's not worth testing the code 
> paths that will invoke a custom __displayhook__ or
> __excepthook__, but I regret it nevertheless. :-)
> maybe this deserves a comment?

Testing a custom displayhook is now done (via compile(...,
"single")/exec). Testing a custom excepthook seems to be
trickier. This could probably be done by calling the
interpreter recursively via os.system() or os.popen(). I've
added a comment for now that this isn't tested.
Unfortunately this leaves a large block in
Python/pythonrun.c uncovered.

> - sys.exit() should also be callable with a string

OK, done.

> - you could check that the value of the SystemExit exception
> has the right exit code

Done.

- Have you checked this with Jython? I don't know if it
implements all
   of these; in particular I doubt it has getrefcount().

I haven't tested Jython yet, but I guess test_sys.py will
have to many many exceptions for Jython. I'll try this tomorrow.

- I presume you've tested this on Windows?

Linux & Windows