Initial Python 3 module port

[itamarst]

Port a single module and its tests to Python 3, probably allmydata.util.humanreadable.

The main goal is not so much the port as starting the infrastructure:

• Help with writing up a checklist and process.
• Set up the tracking infrastructure (a module to list which modules have been ported).

In order to be merged, the Python 3 testing ratchet should be increased to include any new passing tests.