code: def _get_v4_signed_headers(self):
          
          if self.aws_session is None:
              boto_session = session.Session()
              creds = boto_session.get_credentials()
          else:
              creds = self.aws_session.get_credentials()
          if creds is None:
              raise CerberusClientException("Unable to locate AWS credentials")
          readonly_credentials = creds.get_frozen_credentials()
  
          # hardcode get-caller-identity request
          data = OrderedDict((('Action','GetCallerIdentity'), ('Version', '2011-06-15')))
          url = 'https://sts.{}.amazonaws.com/'.format(self.region)
          request_object = awsrequest.AWSRequest(method='POST', url=url, data=data)
  
          signer = auth.SigV4Auth(readonly_credentials, 'sts', self.region)
          signer.add_auth(request_object)
          return request_object.headers
docs: Returns V4 signed get-caller-identity request headers