Backup the current config file.

def backup_config(self, filename):
        """Backup the current config file.
        """
        backup_name = filename + '-' + self.version
        warn('Moving current configuration to ' + backup_name)
        try:
            shutil.copy2(filename, backup_name)
            return True
        except (IOError, OSError) as e:
            print('Error copying %s: %s' % (filename, e.strerror or e), file=sys.stderr)
            return False