editor: commit to Python 3.

editor: commit to Python 3.

At the time I wrote this editor, it was sensible to try to be Python
2/3 agnostic. P2 is now thoroughly obsolete, so I've removed the P2
affordances. That lets me fix the shebang line to say 'python3', and
chmod the file +x, so that you can run it by its name.

Also, it can be renamed to just 'editor', because now that it's
executable, the fact that it's in Python is nothing but an internal
implementation detail.