Build: update setuptools config, package structure and CI

[MuellerSeb]

Changes:

• drop Python 3.6 support (setuptools>60 needs py>3.7)
• move setup.cfg content to pyproject.toml (PEP 621)
• update cibuildwheel to >2.6 (requirement for pyproject.toml config with setuptools)
• move to src/ based package structure (better for testing, building and structure)
• build wheels for apple silicon
• apply isort and add check to CI
• fix documentation