docs: def raise_for_status(self):
          '''Raise Postmark-specific HTTP errors. If there isn't one, the
          standard HTTP error is raised.
  
          HTTP 401 raises :class:`UnauthorizedError`
  
          HTTP 422 raises :class:`UnprocessableEntityError`
  
          HTTP 500 raises :class:`InternalServerError`
          '''
          if self.status_code == 401:
              raise UnauthorizedError(self._requests_response)
          elif self.status_code == 422:
              raise UnprocessableEntityError(self._requests_response)
          elif self.status_code == 500:
              raise InternalServerError(self._requests_response)
          return self._requests_response.raise_for_status()