Add a cbd_subject() method

[Natureshadow]

Summary of changes

This PR adds a new Graph.cbd_subject() method. It is the counter-part to Graph.cbd(): It checks whether the graph is a CBD for some resource and, if it is, returns the subject of that resource.

I need this for implementing federation between graphs, where incoming pushes shall be ckecked to be CBDs because CBDs are nice to test against ACLs (cf. https://socialhub.activitypub.rocks/t/formalizing-inbox-posts-to-be-cbds/3123)

It also adds a Graph.roots() method returning the roots of the graph, used as a utility in cbd_subject, but also useful otherwise.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• For changes that have a potential impact on users of this project:
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
  • Considered adding an example in ./examples for new features.
  • Considered updating our changelog (CHANGELOG.md).
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage: 90.865% (+0.01%) from 90.854% when pulling 76fbb4e on Natureshadow:is-cbd into e103078 on RDFLib:main.

[aucampia]

Thanks for the PR @Natureshadow, as this is an enhancement I will want some feedback from other maintainers also, and I will still think about it a bit and maybe extend the docstrings a bit just to make sure I understand the intent.

[ghost]

Interesting use case, can see your requirement and its potential generality. Also nice to see RDFLib being used to contribute to the development of the fediverse and impressed that Melvin is both taking an interest and endorsing your stance.

[aucampia]

PRs to V6 is closed until further notice. See this for more details:

• RDFLib V6 and V7 #2395

[aucampia]

PRs to V6 is closed until further notice. See this for more details:

• RDFLib V6 and V7 #2395

We will be open for PRs again once this is resolved:

• docs: add guidelines for breaking changes #2402

[aucampia]

Graph.cbd_subject() is a very special case I think, it may be useful in some select circumstances, but I think it will be the first thing we drop if we ever rework things as the surface area of RDFLib is already way too big for the maintenance resources. I will be open to integrating something like this in rdflib._contrib where we don't have any API guarantees.

I will leave this open but as of right now my main priority is improving quality.