Use tobytes() instead of tostring()

[supersonicclay]

Fixes #418

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[supersonicclay]

I think I had a previous username on the CLA. It should be updated to supersonicclay now. Can you re-run the CLA check?

[rjhuijsman]

Hey folks! This bug means that your nice sample packages can't be used with Python 3.9. Can we do something to get this fix merged?