code: def from_string(cls, key, key_id=None):
          
          key = _helpers.to_bytes(key)
          private_key = serialization.load_pem_private_key(
              key, password=None, backend=_BACKEND)
          return cls(private_key, key_id=key_id)
docs: Construct a RSASigner from a private key in PEM format.
  
          Args:
              key (Union[bytes, str]): Private key in PEM format.
              key_id (str): An optional key id used to identify the private key.
  
          Returns:
              google.auth.crypt._cryptography_rsa.RSASigner: The
              constructed signer.
  
          Raises:
              ValueError: If ``key`` is not ``bytes`` or ``str`` (unicode).
              UnicodeDecodeError: If ``key`` is ``bytes`` but cannot be decoded
                  into a UTF-8 ``str``.
              ValueError: If ``cryptography`` "Could not deserialize key data."