[Twisted-Python] Python 3: Odd failure to print unhandled error tb

[Jason Litzinger]

> My guess is that since `d` is at the global scope, it doesn't get garbage collected until some unfortunately late phase in the interpreter tear-down where maybe stderr isn't even available any more.
Here be dragons.
> 
> I'd definitely ignore this case for now and fix the others, which are much more serious, and tractable to fix ;).
That's where I've been leaning, so I'll avoid this case for now.  I'll
probably reference this in #7927 for completeness.

Thanks!
-Jason