Replace all remaining six usage with native Python 3 equivalents (#7704)

changelog.d/7704.misc

contrib/graph/graph3.py

scripts-dev/federation_client.py

scripts/synapse_port_db

setup.cfg

synapse/_scripts/register_new_matrix_user.py

synapse/api/errors.py

synapse/api/filtering.py

synapse/api/urls.py

synapse/appservice/__init__.py

synapse/appservice/api.py

synapse/config/_base.py

synapse/config/appservice.py

synapse/config/tls.py

synapse/crypto/keyring.py

synapse/events/utils.py

synapse/events/validator.py

synapse/federation/federation_base.py

synapse/federation/federation_server.py

synapse/federation/transport/client.py