List/get needed modules in `setup.cfg`

[mhvk]

Don't know if this is easy, but it would seem that the list of modules for which to list versions should just be gotten from setup.cfg - it could then default to install_requires or something else that is sensible.

[astrofrog]

This is easy to do! See astropy/astropy#8867 - but that idea met opposition so I gave up. Let's discuss more in astropy/package-template#438 (comment) and compare to alternatives?