> Number of OS threads to spawn.

Ah I see, so this would correspond with the "max_workers" argument of ThreadPoolExecutor then, correct? If so, we could pass this in the __init__ for ThreadPool:

def __init__(self, concurrency):
    ...
    self._executor = concurrent.futures.ThreadPoolExecutor(max_workers=concurrency)

IMO, I think it would be a bit more clear to just explicitly call it "threads" or "max_threads", as that explains what it's effectively doing. While "concurrency" is still a perfectly correct way of describing the behavior, I think it might be a little too abstract for an argument name.