Add a cancelling_pending async context manager #39

@asynccontextmanager
async def cancelling_pending(*futures: Future[Any]):
    try:
        yield
    finally:
        for future in futures:
            if not future.done():
                future.cancel()
                try:
                    await future
                except asyncio.CancelledError:
                    pass

class Timer:
    ...
    async def ready() -> None:
        ...
        while time_to_next_tick > 0:
            tasks = [
                asyncio.create_task(asyncio.sleep(time_to_next_tick)),
                asyncio.create_task(self._reset_event.wait()),
            ]
            async with cancelling_pending(tasks):
                await next(asyncio.as_completed(tasks))
            self._reset_event.clear()
            now = self._now()
            time_to_next_tick = self._next_tick_time - now

            _, pending = await asyncio.wait(
                [
                    asyncio.create_task(asyncio.sleep(time_to_next_tick)),
                    asyncio.create_task(self._reset_event.wait()),
                ],
                return_when=asyncio.FIRST_COMPLETED,
            )
            for task in pending:
                task.cancel()
                try:
                    await task
                except asyncio.CancelledError:
                    pass