docs: def _get_headers(self, api_key=None, test=None, request_args=None):
          '''Constructs the headers to use for the request.
  
          :param api_key: Your Postmark API key. Defaults to `None`.
          :param test: Use the Postmark test API. Defaults to `self.test`.
          :param request_args: Keyword args to pass to :func:`requests.request`.
              Defaults to `None`.
          :rtype: `dict` of header values.
          '''
          if request_args is None:
              request_args = {}
          headers = {}
          headers.update(self._headers)
          headers.update(request_args.pop('headers', {}))
          if (test is None and self.test) or test:
              headers[self._api_key_header_name] = POSTMARK_API_TEST_KEY
          elif api_key is not None:
              headers[self._api_key_header_name] = api_key
          else:
              headers[self._api_key_header_name] = self.api_key
          if not headers.get(self._api_key_header_name):
              raise ValueError('Postmark API Key not provided')
          return headers