In the process of porting an old project from the now defunct d2to1 I noticed a feature of d2to1 that is still missing from setuptools is support for configuring extension modules in setup.cfg.

At least, that's what my reading of the source suggests, especially ConfigOptionsHandler

In d2to1 this looks something like:

[extension=<module_name>]
sources = <file1> <file2> ...
language = c
<keyword> = <value>

where each key/value pair correspond to setuptools.Extension arguments.

For setuptools it might look something more like:

[options.ext_modules.<module_name>]
<key>=<value>

Though I'm open to other suggestions for the format, and would be happy to provide a PR once we agree on and nail down a format.