resolve issue1868, add a method to expand qname to URI

[ghost]

Summary of changes

In response to issue #1868:

“I sometimes need use the reverse of qname() to make the URI input more human-friendly. It would be great if we have something like resolve_qname() which would translate a qname ns:obj back to http://example.com/ns#object”

Seems a reasonable enough request and trivial to resolve... add NamespaceManager method expand_qname and tests.

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.

[aucampia]

Happy to make the suggested changes if you want.

[aucampia]

I'm also a bit on the fence on returning None, I would just raise an error, but I would not say this is wrong, would like to hear some other opinions on the matter.

CC: @RDFLib/core-reviewers

[nicholascar]

I prefer raising an error: if the thing asked for cannot be calculated, an error should be returned, not None which just indicates it doesn't exist. This is a calculation function, not a look up value function.

[aucampia]

Made a PR against your branch changes logic slightly to be more compliant with CURIE spec:

• https://github.com/gjhiggins/rdflib/pull/6

[aucampia]

Thanks @gjhiggins, very useful method to have.

[edmondchuc]

Nice, looks good.

[dany-nonstop]

many thanks @gjhiggins for answering the feature request and adding this new feature. It works beautifully.