docs: def cancel(self, username, project, build_num):
          """Cancel the build and return its summary."""
          method = 'POST'
          url = ('/project/{username}/{project}/{build_num}/cancel?'
                 'circle-token={token}'.format(username=username,
                                               project=project,
                                               build_num=build_num,
                                               token=self.client.api_token))
          json_data = self.client.request(method, url)
          return json_data