Fix docs CI build and setuptools

[guillermoaguilar]

Docs were not being built because jupyter-book changed their way of parsing docs since version 2.0. Our docs use the old format used for versions <2.

This PR fixes the version number to jupyter-book>=1.0,<2 in docs/requirements.txt and in the docs extra of pyproject.toml (which wrongly requested 0.14).

Three other fixes:

• docs/requirements.txt listed packages as YAML (- sphinx), which was not read properly by pip.
• pyproject.toml declared no packages, now explicitely declares psignifit*.
• psignifit/__init__.py imported os and subprocess but neither were used.