docs: def load_cert_chain(self, certfile, keyfile=None):
          """Load a private key and the corresponding certificate. The certfile
          string must be the path to a single file in PEM format containing the
          certificate as well as any number of CA certificates needed to
          establish the certificate's authenticity. The keyfile string, if
          present, must point to a file containing the private key. Otherwise
          the private key will be taken from certfile as well.
          """
          self._certfile = certfile
          self._keyfile = keyfile