Deprecate ConjunctiveGraph (#2405)

[wallberg]

Summary of changes

Deprecate ConjunctiveGraph, in favor of Dataset, as per #2405 .

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• [] All tests pass, but lint and mypy do not, I believe due to pre-existing issues.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[wallberg]

All tests are passing, but this introduces a large number of new warnings about the deprecation. Should I attempt to suppress all of those warnings, as part of this PR?

[nicholascar]

@wallberg thanks for this PR, it's been a long tme coming (5 years, 3 RDFLib major versions...).

We have some issues with PRs passing validation just at the moment. When I return from leave, next week, I will try and fix this issue. Then we will need to see this PR pass the test suite again. If you had it passing locally, it likely will pass fine as the errors we are seeing are, I think, environment issues, not code errors.

So please hold tight for a couple of weeks on this and I we will get this PR through likely in June. I'm really looking forward to greatly improving RDFLib's multi-graph support and this is part of the whole task.