Currently the only way to use a class other than `threading.Thread` with `concurrent.futures.ThreadPoolExecutor` is to extend `ThreadPoolExecutor` and override the private method `_adjust_thread_count()`. 

For example, suppose I have a class that applies some custom logic when running code in a new thread:

```
class MyThread(threading.Thread):
    def run(self):
        with some_important_context():
            super().run()
```

Using this class with `ThreadPoolExecutor` requires me to write the following code:

```
class MyThreadPoolExecutor(ThreadPoolExecutor):
    def _adjust_thread_count(self):
        # if idle threads are available, don't spin new threads
        if self._idle_semaphore.acquire(timeout=0):
            return

        # When the executor gets lost, the weakref callback will wake up
        # the worker threads.
        def weakref_cb(_, q=self._work_queue):
            q.put(None)

        num_threads = len(self._threads)
        if num_threads < self._max_workers:
            thread_name = '%s_%d' % (self._thread_name_prefix or self,
                                     num_threads)
            t = MyThread(name=thread_name, target=_worker,
                         args=(weakref.ref(self, weakref_cb),
                               self._work_queue,
                               self._initializer,
                               self._initargs))
            t.start()
            self._threads.add(t)
            _threads_queues[t] = self._work_queue


with MyThreadPoolExecutor(max_workers=1) as executor:
    future = executor.submit(pow, 323, 1235)
    print(future.result())
```

That's a bummer, because now I have to maintain this method if there are upstream fixes/changes. I also can't count on it existing in the future, since it's a private method. 

In other words, `ThreadPoolExecutor` is not composable, and extending it to use a custom `Thread` class is neither safe nor maintainable.

Here's what I'd like to be able to do instead:

```
with ThreadPoolExecutor(max_workers=1, thread_class=MyThread) as executor:
    future = executor.submit(pow, 323, 1235)
    print(future.result())
```