Just-jobs

Changed

- Deprecated `.now` in favor of direct calling (`func()`).

I discovered arq, and it does a lot of the same things I was hoping just-jobs would eventually support. Rather than remake the wheel, V2 is a complete rethinking and rewrite using arq as an engine.

Some internal concepts are the same though:

- Redis is the default broking service.
- You can use both sync and async jobs.
- Sync jobs are run in an executor to prevent event loop blocking.
- Jobs are runnable if you don't want to delay it.

Changed license to BSD-3-Clause.

Changed

- Replaced `aioredis` with asyncio `redis-py` and `hiredis`.
- Replaced `pdoc3` with `pdoc`.

Removed

- Extraneous `anyio` dependency.
- Support for Python <3.7.