Refactor build system

[almarklein]

Closes #585.

The approach to use cibuildwheel is somewhat overkill because we don't need to compile anything. And it does not even work properly. We added extra logic to make it work in #582 but then we still got #585. So I figured it can be done simpler, using the modern approach of a pyproject.toml.

• Use pyproject.toml
• Remove setup.py, dev-requirements.txt
• Setup build system so it creates clean sdist and wheels.
• Enable cross-platform builds.
• Script to build all wheels. The wheels are checked extensively.
• Update cd.yml, test wheels on 3 main platforms.