Fix flake8 (#4519)

changelog.d/4519.misc

synapse/_scripts/register_new_matrix_user.py

synapse/handlers/directory.py

synapse/handlers/federation.py

synapse/push/clientformat.py

synapse/storage/__init__.py

synapse/storage/events.py

synapse/storage/events_worker.py

tests/storage/test_background_update.py

tests/storage/test_end_to_end_keys.py

tests/storage/test_keys.py

tests/storage/test_state.py