Explicit `#!2` and `#!3` snippets for python{2,3}

Reasoning:

Different distros handle the symlinks to `python` differently:

- On Debian-like it is linked to `python2`
- On Arch it is linked to `python3`

Plus 2 needs an encoding line, 3 doesn’t.

You generally want to explicitely specify which Python version the code is in.