setup.py build_ext alternatives?

[dweindl]

For building the Python extensions during model import, we have always been relying on setup.py build_ext. However, invoking python setup.py is deprecated since a while (https://setuptools.pypa.io/en/latest/deprecated/commands.html).

We should find a proper replacement here. Either through the setuptools API, the cmake-build-extension API, or less elegantly by building wheels via build and extracting the extension.