drop __future__ and future imports

src/_zkapauthorizer/_base64.py

src/_zkapauthorizer/_plugin.py

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/_storage_server.py

src/_zkapauthorizer/configutil.py

src/_zkapauthorizer/controller.py

src/_zkapauthorizer/eliot.py

src/_zkapauthorizer/foolscap.py

src/_zkapauthorizer/lease_maintenance.py

src/_zkapauthorizer/model.py

src/_zkapauthorizer/private.py

src/_zkapauthorizer/resource.py

src/_zkapauthorizer/schema.py

src/_zkapauthorizer/server/spending.py

src/_zkapauthorizer/spending.py

src/_zkapauthorizer/storage_common.py

src/_zkapauthorizer/tests/eliot.py

src/_zkapauthorizer/tests/fixtures.py

src/_zkapauthorizer/tests/foolscap.py

src/_zkapauthorizer/tests/json.py