[mod] Use pathlib module in exemples and a test

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

tests/key/test_signing_key.py