code: def update_compatibility(self, level, subject=None):
          
          if level not in VALID_LEVELS:
              raise ClientError("Invalid level specified: %s" % (str(level)))
  
          url = '/'.join([self.url, 'config'])
          if subject:
              url += '/' + subject
  
          body = {"compatibility": level}
          result, code = self._send_request(url, method='PUT', body=body)
          if code >= 200 and code <= 299:
              return result['compatibility']
          else:
              raise ClientError("Unable to update level: %s. Error code: %d" % (str(level)), code)
docs: PUT /config/(string: subject)
  
          Update the compatibility level for a subject.  Level must be one of:
  
          :param str level: ex: 'NONE','FULL','FORWARD', or 'BACKWARD'