[lint] Use sys.exit() instead of exit()

examples/save_and_load_private_key_file.py

examples/save_and_load_private_key_file_ewif.py

examples/save_and_load_private_key_file_pubsec.py

examples/save_and_load_private_key_file_wif.py

examples/save_revoke_document.py

examples/send_certification.py

examples/send_transaction.py