I don't think you need slicing if you rewrite the patch in another way, e.g.:

for i in range(n):
    try:
        waiter = __waiters.popleft()
    except IndexError:
        break
    waiter.release()

I think this is safe, since notify() must be called with the lock held: another thread shouldn't be able to mutate the waiters list in the meantime.

As for notify_all(), it could be optimized to swap the internal list with an empty one: there's no need to pop the waiters one by one.