Setuptools is required at run-time, not just build-time

[adamjstewart]

Previously, we relied on the fact that setuptools must be installed at build-time to ensure that it was also available at run-time. However, unless setuptools is explicitly listed as a run-time dependency, there's no guarantee that it won't be uninstalled. Package managers like Spack only add run-time dependencies to PYTHONPATH, so this distinction becomes even more important.

I'm not sure what versions of setuptools are required, but let me know if you want me to add an upper/lower bound.

[rbuffat]

As far as I understand there should be no run-time dependency of Fiona on setuptools.

[adamjstewart]

There's one right here: https://github.com/Toblerity/Fiona/blob/1.9.0/fiona/fio/main.py#L7

[sgillies]

Yes, that's it, CLI command discovery depends on setuptools's pkg_resources. If you install fiona with pip or use conda, you'll have pkg_resources (for now), which is why our tests continued to pass, but that's not the case for Spack.