Fix trix parser

[ghost]

Summary of changes

Changed TriX parser to allow trix and TriX

The RDFLib TriX parser currently only accepts TriX documents conforming to the the Nokia-published XSD which specifies (the mixed-case) TriX, contradicting the W3C-published XSD spec for TriX which specifies (the lower-case) trix. We should accept both.

We were also a bit light on a TriX test suite for exercising the parser, so I recruited some TriX test fixtures from NG4J and Jena and fleshed out the TriX test suite, using the W3C "Manifest"-style approach.

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

[coveralls]

Coverage increased (+0.06%) to 88.501% when pulling cc12c4c on gjhiggins:fix-trix-parser into a3a4611 on RDFLib:master.

[aucampia]

Fix looks good and nice to have the additional tests, just some minor notes on naming.

[aucampia]

Looks good to me.

[aucampia]

Will merge by 2022-05-25 if there is no further feedback. This is a very simple fix.