Run a command in a subshell.

def _execute(cmd):
        """Run a command in a subshell."""
        cmd_parts = shlex.split(cmd)
        if sys.version_info[0] < 3:

            cmd_parts = shlex.split(cmd.encode('ascii'))

        proc = subprocess.Popen(
            cmd_parts,
            stdout=subprocess.PIPE,
            stderr=subprocess.PIPE,
        )
        out, err = proc.communicate()
        if proc.returncode != 0:

            raise subprocess.CalledProcessError(
                returncode=proc.returncode,
                cmd=cmd,
                output=err,
            )

        return CommandResult(
            code=proc.returncode,
            out=out.decode('utf8'),
            err=err.decode('utf8'),
        )