Convert DER octet string to PEM format (with optional header)

def der2pem(der_string, obj="UNKNOWN"):
    """Convert DER octet string to PEM format (with optional header)"""
    # Encode a byte string in PEM format. Header advertizes <obj> type.
    pem_string = ("-----BEGIN %s-----\n" % obj).encode()
    base64_string = base64.b64encode(der_string)
    chunks = [base64_string[i:i + 64] for i in range(0, len(base64_string), 64)]  # noqa: E501
    pem_string += b'\n'.join(chunks)
    pem_string += ("\n-----END %s-----\n" % obj).encode()
    return pem_string