Build: update setuptools config 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)
• use extension-helpers for openmp support in setup.py

[MuellerSeb]

OpenMP support was never available in CI for macOS. There is also an open PR for clang/openmp support on macOS here: astropy/extension-helpers#42

We could wait for this PR to be merged and then we could install openmp with brew for macOS:

      - name: macos openmp install
        uses: mstksg/get-package@v1
        if: matrix.os == 'macos-latest'
        with:
          brew: libomp

[LSchueler]

That's a nice improvement in the setup! A small fix in the toml-file and you're ready to merge this 👍

[MuellerSeb]

When this is implemented, we can even drop the MANIFEST.in: pypa/setuptools#3341