Eventually deprecate `setup.cfg` with automatic conversion to `pyproject.toml`

This is a follow up of #1688.

As discussed in the linked issue, 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 https://github.com/pypa/setuptools/issues/1688#issue-411224984:

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 https://github.com/pypa/setuptools/issues/1160, and if we decide on that option we can move over to that issue).

Originally posted by Jason R. Coombs in https://github.com/pypa/setuptools/issues/1688#issuecomment-471212342

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.

Originally posted by Paul Ganssle in https://github.com/pypa/setuptools/issues/1688#issuecomment-570933225

I don't think we should warn if people use setup.cfg even when pyproject.toml is available, we should wait for pyproject.toml configuration to be stable for at least a year before actively pushing it like that.

Relevant tool: ini2toml

Start by reading the linked issue #1688 and the referenced ini2toml documentation. The issue does not name implementation files or tests; completion would require an agreed approach for converting setup.cfg to pyproject.toml and deprecating the former.