Add pyproject.toml

[MuellerSeb]

This PR adds a pyproject.toml and updates the installation setup.

• rely on pep517/518 builds for wheels and sdist
• removed all requirements*.txt files (in setup.cfg now)
• use setup.cfg for all setup configurations
• use setup.py only for cython and openmp configurations
• remove install options for openmp and use an environment variable GSTOOLS_BUILD_PARALLEL (like sklearn) instead
• use pyproject.toml to define build dependencies, setuptools_scm, coverage and black
• add format checking stage in CI (using black)
• update CI script to always use pip and the extras defined in setup.cfg ([test])
• add .readthedocs.yml to install gstools with pip and [doc] extras on readthedocs
• updated MANIFEST.in to shrink wheels and sdist (from 45mb to 28mb in the artifacts on GH-Actions)
• removed development extra [dev] from setup which is equal to pip install gstools[test, doc]

[LSchueler]

I only had a quick look, but this looks so much cleaner! Approved! ;-)