Use `concurrent.futures` module for running things in parallel

The simplest way to run something in multiple threads/processes is through the [`concurrent.futures`](https://docs.python.org/3/library/concurrent.futures.html) module instead of 'manually' managing the threads/processes as currently done in [https://carpentries-incubator.github.io/lesson-parallel-python/threads-and-processes.html. Therefore it might be nice to ](https://github.com/carpentries-incubator/lesson-parallel-python/blob/f839f0f9dfa5d81e163775a127e7d7e8675ff114/episodes/threads-and-processes.md?plain=1#L28-L38)

Therefore, it might be nice to use the `concurrent.futures` module instead.