x11/libinput: simplify port

- drop unnecessary OLD_CMD (this is one of the standard replacements,
  at least in combination with allowing all current Pythons)
- don't add to MESON_ARGS, assign instead (they start out unset)
- allow all relevant Pythons (any of the supported Python 3 versions)
- use HTTPs by default (the http link redirects there anyway)
- shebang all the Python scripts, even the ones not actually installed.