aab75bb1d0
Python scripts should already properly run, whether you run them with a direct python shebang or a `env python` one (cf. previous commit). But it's still nice to install a `.interp` file, which allows to control exactly the interpreter to use, overriding the shebang. With this file, Python scripts will use installation-time Python interpreter. Also update old code to make the interp contents about Python 3 instead of 2.