Fix passing ConjunctiveGraph as namespace_manager

[mielvds]

Summary of changes

• passes the NamespaceManager of the ConjunctiveGraph as namespace_manager of the Graph created by the get_context() method instead of the ConjunctiveGraph object itself.

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 granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage remained the same at 90.447% when pulling 59ca17b on mielvds:patch-1 into cfa4180 on RDFLib:master.

[aucampia]

@mielvds thanks for the PR. While I think it is pretty safe and uncontroversial it would still be good to have tests for this.

Also, we are trying to reduce the effort it takes to make a release, and one part of this is to keep the changelog up to date always:

rdflib/CHANGELOG.md

Lines 201 to 217 in cfa4180

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	- PLACEHOLDER.
	Description of changes.
	Closed [issue #....](https://github.com/RDFLib/rdflib/issues/).
	[PR #....](https://ichard26.github.io/next-pr-number/?owner=RDFLib&name=rdflib).
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So it would be good if you could add tests and update the changelog, but I will try find some time this weekend to add tests and update the change on your PR branch if it is not done yet, as this is rather important to fix.

[aucampia]

@mielvds please share how you managed to trigger this if you can.

[mielvds]

Hi @aucampia
I agree. I will add some tests and update the changelog. You can then check if they suffice.

I triggered this while developing a documentation tool for shacl. I manage different shacl graphs as named graphs and got errors when printing the namespaces of individual graphs.

[aucampia]

Thank you very much for the test and changelog update @mielvds, the change looks good.