Return a PrivateKey instance.

        :param private_key: Private key string (PEM format) or the path
                            to a local private key file.

def load_pem(cls, private_key, password=None):
        """Return a PrivateKey instance.

        :param private_key: Private key string (PEM format) or the path
                            to a local private key file.
        """
        # TODO(sam): try to break this in tests
        maybe_path = normpath(private_key)
        if os.path.isfile(maybe_path):
            with open(maybe_path, 'rb') as pkf:
                private_key = pkf.read()
        if not isinstance(private_key, six.binary_type):
            private_key = private_key.encode('utf-8')

        pkey = serialization.load_pem_private_key(
            private_key,
            password=password,
            backend=crypto_backends.default_backend())
        return cls(pkey)