docs: def discovery(self, url=None):
          """
          Retrieve the standard discovery file that provides routing
          information.
  
          >>> Three().discovery()
          {'discovery': 'data'}
          """
          if url:
              data = self.session.get(url).content
          elif self.discovery_url:
              response = self.session.get(self.discovery_url)
              if self.format == 'xml':
                  # Because, SF doesn't follow the spec.
                  data = xml(response.text)
              else:
                  # Spec calls for discovery always allowing JSON.
                  data = response.json()
          else:
              data = self.get('discovery')
          return data