Typing and Splitting Files for Readability

[afishman-openai]

The goal of this PR is to improve the readability of Urchin. I've often found that it's difficult to grok the codebase because everything lives in a single file and has no types. This PR splits things into separate files based on the type of object it is and adds types.

I think there could be another, future version that involves better typing but this would require a much more significant rewrite. In the meantime, this version will improve things without doing any major surgery.

Since this is still a pretty major internal change, I've ensured that all the existing tests pass without much modification.

Also, this PR drops support for unsupported versions of Python (<3.9). Users of this package that want to keep using these Python versions would have to install an earlier version. Python 3.8 had EOL in late 2024.

[fishbotics]

@traversaro : can you take a look at this? Since it's my PR, I want to make sure you're ok with it before merging.

[traversaro]

@traversaro : can you take a look at this? Since it's my PR, I want to make sure you're ok with it before merging.

Thanks for asking, the PR seems great! My only doubt is the ruff/mypy versions. It seems that the pre-commit is using a relatively old ruff and mypy version, while the CI use the latest version. Is that intentional?

[fishbotics]

Fixed!