packaging: use 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 configurations
• use pyproject.toml to define build dependencies, setuptools_scm, coverage and black
• update CI script to always use pip and the extras defined in setup.cfg ([test])
• add .readthedocs.yml to install pykrige with pip and [doc] extras on readthedocs
• updated MANIFEST.in to shrink wheels and sdist
• removed development extra [dev] from setup which can be achieved with all other extras