Cron jobs timeout after 300 seconds when timeout=None

[Graeme22]

Given the following settings:

async def long_job(ctx):
    # simulate job that takes a long time
    await asyncio.sleep(3600)


class WorkerSettings:
    cron_jobs = [
        cron(long_job, run_at_startup=True)
    ]
    # ...

the job times out after 300 seconds with TimeoutError, even though the timeout is None, which is the default for cron jobs.
I believe the culprit is this line, which replaces the None value although None may be desired.

If desired I can submit a PR.