As discussed in #1688., the approach setuptools would like to take is to eventually use a single declarative format (pyproject.toml) instead of maintaining 2 (pyproject.toml/setup.cfg).

Originally posted by Paul Ganssle in #1688 (comment):

Add pyproject.toml as the one true way to do declarative builds, moving all setuptools configuration over there and deprecating setup.cfg (partially what is discussed in #1160, and if we decide on that option we can move over to that issue).

Originally posted by Jason R. Coombs in #1688 (comment)

One way to ease the transition could be for the setup.cfg code to generate a .toml file and consume then have the pyproject.toml consumer just load both files. That way, a project seeking to transition would only need to do one build, manually merge the two .toml files, and delete setup.cfg.

Relevant tool: ini2toml