Handle custom build_py inheriting from distutils
Handle custom build_py inheriting from distutils

According to #2849, some projects, including important data-science
packages rely on `distutils` when creating custom commands, instead of
extending the ones provided by setuptools.

This change should accomodate this use case, while also warning the
users to migrate.

pypa/setuptools,pypa/setuptools,pypa/setuptools