Astropy Proposal for Enhancement 17: A roadmap for package infrastructure without astropy-helpers (APE 17)

The astropy-helpers package contains a collection of utilities intended to help build and distribute the core astropy package, as well as coordinated and affiliated packages in the Astropy ecosystem, and it is also used by packages outside of the ecosystem (at the time of writing, almost 400 repositories on GitHub appear to use astropy-helpers).

The original APE proposing the creation of astropy-helpers (APE 4) was accepted in June 2014. Since then, the state of the Python packaging ecosystem has changed significantly, with advances such as binary wheels that can be provided for all major platforms, tools such as tox and setuptools_scm which are now stable and mature, and more recently support for PEP 517 and PEP 518 (in pip 19.0 or later), and many incremental updates and new features to setuptools (in setuptools 30.3 or later). The purpose of this APE is to show how by relying on these tools, we can significantly simplify the packaging for astropy and all the affiliated packages and other packages using astropy-helpers, and remove the need for astropy-helpers altogether in pure-Python packages.