[lint] Handle new Pylint rules

duniterpy/api/client.py

duniterpy/documents/transaction.py

duniterpy/helpers/blockchain.py

duniterpy/key/signing_key.py

examples/load_cleartext_ascii_armor_message.py

examples/load_encrypted_ascii_armor_message.py

examples/save_cleartext_ascii_armor_message.py

examples/save_encrypted_ascii_armor_message.py

examples/save_revoke_document.py

tests/key/test_signing_key.py