init: python3.

I've actually left the two other scripts on python2, since they're not
used in the build, and I don't actually know whether anyone's still
using them manually. Leaving them on python2 should be a sign to future
maintainers (assuming they're far enough in the future) that maybe
no-one's using them and they can just be deleted instead.

Test: treehugger
Change-Id: I4d7dfdbf9f2a34ddd15cd4be077eeef2248cc596