Build one wheel.

        :return: The filename of the built wheel, or None if the build failed.

def _build_one(self, req, output_dir, python_tag=None):
        """Build one wheel.

        :return: The filename of the built wheel, or None if the build failed.
        """
        # Install build deps into temporary directory (PEP 518)
        with req.build_env:
            return self._build_one_inside_env(req, output_dir,
                                              python_tag=python_tag)