libinput switched from Doxygen to Sphinx for user docs. Since Sphinx is a Python
module, it propagates Python. And because it is listed in nativeBuildInputs,
its python binary takes precedence over the one added in buildInputs.
This results in a wrong interpreter being substituted into shebangs.

The contamination occurred previously too but libinput does not use pyparsing
dependency since 1.12.0, so it could be removed.

I prevented Sphinx from propagating Python and added some additional
dependencies to it. In the future we might want something more reusable.

While at it, I also fixed the tests.

libinput: fix docs & libinput command