title: Python: Wait on all of `concurrent.futures.ThreadPoolExecutor`'s futures
body: <p>I've given <code>concurrent.futures.ThreadPoolExecutor</code> a bunch of tasks, and I want to wait until they're all completed before proceeding with the flow. How can I do that, without having to save all the futures and call <code>wait</code> on them? (I want an action on the executor.)</p>