respect pyproject.toml

I'm wondering if we can instead of creating a `juliapkg.json`, but putting a field inside `pyproject.toml` like

```toml
[julia.dependencies]
julia = "1.8"
Example = "0.5"
PythonCall = "0.8"
```

this would making packaging a lot easier because `pyproject.toml` will be saved in a package wheel.