code: def _get_credentials(self):
          # type: () -> Optional[Tuple[str, str]]
          
          site = self.data[self.hdx_site]
          username = site.get('username')
          if username:
              return b64decode(username).decode('utf-8'), b64decode(site['password']).decode('utf-8')
          else:
              return None
docs: Return HDX site username and password
  
          Returns:
              Optional[Tuple[str, str]]: HDX site username and password or None