docs: def _get_api_url(self, secure=None, **formatters):
          '''Constructs Postmark API url
  
          :param secure: Use the https Postmark API.
          :param \*\*formatters: :func:`string.format` keyword arguments to
              format the url with.
          :rtype: Postmark API url
          '''
          if self.endpoint is None:
              raise NotImplementedError('endpoint must be defined on a subclass')
          if secure is None:
              secure = self.secure
          if secure:
              api_url = POSTMARK_API_URL_SECURE
          else:
              api_url = POSTMARK_API_URL
          url = urljoin(api_url, self.endpoint)
          if formatters:
              url = url.format(**formatters)
          return url