Fallback to setup_requires to pull in pkgconfig

When installing pyalpm in a virtual environment its dependency on pkgconfig is not resolved early enough (which leads to a failed installation, e.g. https://gitlab.archlinux.org/archlinux/arch-repo-management/-/jobs/25890#L320).

This can seemingly be circumvented by an implicit fallback to setup_requires (as is done by python-lz4).

I'll try to provide a MR that fixes this.