Add __all__ to __init__.py

[ThirVondukr]

Implicitly re-export archrule in __init__.py, this is generally a good practice and some linters require that (flake8/ruff and mypy)

[jwbargsten]

I checked it shortly, and indeed it seems to be considered best practice

To better support introspection, modules should explicitly declare the names in their public API using the all attribute. Setting all to an empty list indicates that the module has no public API.

so LGTM

[jwbargsten]

Thanks!