[doc] Update copyright to 2021

duniterpy/__init__.py

duniterpy/api/bma/__init__.py

duniterpy/api/bma/blockchain.py

duniterpy/api/bma/network.py

duniterpy/api/bma/node.py

duniterpy/api/bma/tx.py

duniterpy/api/bma/ud.py

duniterpy/api/bma/wot.py

duniterpy/api/bma/ws.py

duniterpy/api/client.py

duniterpy/api/endpoint.py

duniterpy/api/errors.py

duniterpy/api/ws2p/__init__.py

duniterpy/api/ws2p/network.py

duniterpy/api/ws2p/requests.py

duniterpy/constants.py

duniterpy/documents/__init__.py

duniterpy/documents/block.py

duniterpy/documents/block_uid.py

duniterpy/documents/certification.py