code: def find_first_version(self):
          
          for version in self.versions_to_try:
              remote_url = self.get_download_url(version=version)
              if http_url_exists(remote_url):
                  return version, remote_url
          return None, None
docs: Finds the first version of igraph that exists in the nightly build
          repo from the version numbers provided in ``self.versions_to_try``.