From what I understand in PEP 345, "Requires-Dist" is the name in PKG-INFO files for a distribution's dependencies on other distributions.

setuptools/distutils has never implemented "Requires-Dist" from PEP345 for PKG-INFO.

Otoh, wheel has, combined with PEP376, so you'll see it in *.dist-info METADATA files, when installing from wheels.

install_requires is the name of the keyword argument to setup() that specifies a dependency on another distribution.

yes

Sanity-check: This doesn't mean that one should call setup() with a "required_dist" keyword argument, right?

right

As I understand it. the setup.cfg revolution never happened,

right, see http://packaging.python.org/en/latest/future.html

Also, I just created a trivial setup.py that contains an "install_requires" and doing "python setup.py develop --user" does seem to have created a PKG-INFO file, but it doesn't have a "Requires-Dist" line.

look for the "requires.txt" file. the dependency is in there.