Add __version__ attribute to the package

[adedaran]

Currently
sliceline.__version__ outputs an AttributeError.

Goal
Keep the version management simple with one unique source of truth. Add the __version__ attribute to the package.
See an implementation of BumpVer.

Note
The version number is managed with GitHub release number (i.e. commit tag). The version property in pyproject.toml is just a placeholder that is modify by the Release job in GitHub Action CI.