Fix for issue 893

[ghost]

As iddan reports in issue #893 “Dataset is unpickeld as ConjunctiveGraph because it extends ConjunctiveGraph and inherits the __reduce__() method. It should be unpickled as Dataset instead.”

A fix was provided but unfortunately overlooked. Rectifying this oversight.

[ghost]

Um. Needs some investigation. Looks like the fix works but the test needs some refinement.

[ghost]

Probably don't need to keep the test as part of the standard suite, can be removed later, it's not as if it's going to be subject to regression.

[aucampia]

Seems this is failing sporadically:

• https://github.com/RDFLib/rdflib/runs/4501225806?check_suite_focus=true
• https://github.com/RDFLib/rdflib/runs/4505918939?check_suite_focus=true

[ghost]

Seems this is failing sporadically:

* https://github.com/RDFLib/rdflib/runs/4501225806?check_suite_focus=true

* https://github.com/RDFLib/rdflib/runs/4505918939?check_suite_focus=true

Hmm. Investigating.

[nicholascar]

Yes, I thought it was fine to merge as tests passed but then I too notice the intermittent failing.

If we can sort this out shortly, I'll release 6.0.3 with what we have. There seems to be no end of new, very good, PRs but I want a 6.0.3 out before the work on Graph IDs that @gjhiggins is leading as that's a large item and it would be safe to make a stable release just before it.

[aucampia]

I think this does fix it: #1504 - and makes sense as a cause for the sporadic failure.