Add Python 3 support

[legoktm]

This was intially generated by the automatic 2to3 converter, and then cleaned up by hand. It should be fully compatible with both Python 2.7 and Python 3+ (I tested on both).

print() is now a function, octal literals need an 'o' in them, and subprocess.check_output() returns bytes, which needs to be decoded into a str before comparison.

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Python 2 goes EOL in a little over 6 months (https://pythonclock.org/), so I thought it would be a good time to add Python 3 support :)

[legoktm]

Downstream Debian bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=937264

[legoktm]

@micahflee anything I can do to move this along?

[fionn]

I'm keen to see this happen too. If there's any extra work needed, I'm happy to help out.