tox must be in optional dependencies

[item4]

https://github.com/gawel/aiocron/blob/master/pyproject.toml#L28-L29

Since v2.0, tox and tox-uv is in common dependencies but tox is only need for testing, not required for common usage.

So, when I install aiocron==2.0, install others togather.

[gawel]

2.1 is out to fix that