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.