fix: add a pyproject.toml default for load_toml

[henryiii]

Close #843. I was worried about nox's handling of the CWD but I think this is actually safe.

[paduszyk]

@henryiii Actually, aliasing the load_toml function to load_pyproject (e.g. via functools.partial) could also be an idea...

[henryiii]

I'd rather keep the API surface minimal, I think? If the default is not obvious, we can also leave it, since .load_toml("pyproject.toml") is only marginally longer and doesn't require increasing the API to learn / use.

[edgarrmondragon]

I see no harm in doing this! 👍