remove six and future dependency, also isort and black

setup.cfg

src/_zkapauthorizer/_json.py

src/_zkapauthorizer/_plugin.py

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/_storage_server.py

src/_zkapauthorizer/config.py

src/_zkapauthorizer/configutil.py

src/_zkapauthorizer/controller.py

src/_zkapauthorizer/foolscap.py

src/_zkapauthorizer/lease_maintenance.py

src/_zkapauthorizer/model.py

src/_zkapauthorizer/resource.py

src/_zkapauthorizer/spending.py

src/_zkapauthorizer/tests/storage_common.py

src/_zkapauthorizer/tests/strategies.py

src/_zkapauthorizer/tests/test_client_resource.py

src/_zkapauthorizer/tests/test_controller.py

src/_zkapauthorizer/tests/test_lease_maintenance.py

src/_zkapauthorizer/tests/test_model.py

src/_zkapauthorizer/tests/test_plugin.py