Replace iteritems/itervalues/iterkeys with native versions. (#7692)

changelog.d/7692.misc

synapse/api/auth.py

synapse/api/errors.py

synapse/app/homeserver.py

synapse/events/__init__.py

synapse/events/snapshot.py

synapse/federation/federation_base.py

synapse/federation/federation_server.py

synapse/federation/send_queue.py

synapse/federation/sender/__init__.py

synapse/handlers/appservice.py

synapse/handlers/device.py

synapse/handlers/e2e_keys.py

synapse/handlers/e2e_room_keys.py

synapse/handlers/federation.py

synapse/handlers/groups_local.py

synapse/handlers/message.py

synapse/handlers/pagination.py

synapse/handlers/presence.py

synapse/handlers/room.py