Consider migrating to tox

`tox` support running tests in multiple envs in parallel, whereas, `nox` doesn't, which is painful with large test matrices like this one:

```python
@nox.session(python=["3.8", "3.9", "3.10", "3.11", "3.12"])
@nox.parametrize("pydantic", ["1.10.17", "2.8.2"])
@nox.parametrize("httpx", ["0.25.0", "0.27.0"])
@nox.parametrize("openai", ["1.0.0", "1.51.0"])
```

https://github.com/wntrblm/nox/issues/544#issuecomment-1631849578