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

[thehappycheese]

Thankyou for posting this issue. As someone tinkering on the bleeding edge of packaging tools I have been so confused by the various PEPs and documentation. I thought pyproject.toml was the future but could not find anything definitive to suggest setuptools were going to deprecate setup.cfg.

[tpvasconcelos]

AFAIK the goal is to align on self references at a higher level, such that you don’t have to depend on parser specific features like configparser’s interpolation. Something like:

 [options.extras_require]
 all =
-     %(retrieval)s
-     %(postprocessor)s
-     %(api)s
-     %(validation)s
+     self[retrieval]
+     self[postprocessor]
+     self[api]
+     self[validation]
 retrieval =
-     %(scipy)s
-     %(numpy)s
+     self[scipy]
+     self[numpy]
     pandas>=0.24.1
     rapidfuzz>=0.14.1
 postprocessor =
-     %(pagexml)s
+     self[pagexml]
     jsonschema>=2.6.0
 api =
-     %(postprocessor)s
+     self[postprocessor]
     flask-restx>=1.0.3
 validation =
-     %(numpy)s
+     self[numpy]
     schwifty>=2018.9.1
 scipy =
     scipy>=1.3.0
 numpy =
     numpy>=1.21.2
 pagexml =
     pagexml-slim>=2022.4.12