[Distutils] Custom installation commands in setup.py

Are custom installation commands in setup.py no longer respected by setuptools? For example, the pybind11 project has a custom InstallHeaders command class in its setup.py, which is passed to the setup() call. When setup is imported from setuptools, the custom command class never gets invoked. When setup is imported from distutils.core, the custom command class is invoked. What's the reason for the disparity - can someone please enlighten me? Regards, Vinay Sajip