using `data_files` dict prefixed with
`SITE_PACKAGES = os.path.join(get_python_lib(), "keystone")` is not
portable, see #235 and #297 for details. Regular setup configuration does
not work, e.g., specifying `package_data` etc, since that only affects
`sdist`.

The fix is a hack: customize `install_lib` to explicitly copy the file
into keystone install dir.