Migrate to python3

[dkirkby]

The pre-processed corpus is in unicode after #8 (with utf8 encoding, by default), which means that clues are now given in unicode. The new engine.say() function now handles encoding of all game output, including clues, but we now have a mixture of byte and unicode literals in engine.py that would be much cleaner in python3. This issue is to perform and test the migration.