RDFLib version ceiling should be expanded to <8.0.0 after pySHACL does so

[ajnelson-nist]

RDFlib released 7.0.0 last week.

#116 had pinned case-utils to use rdflib < 7. I tried testing rdflib's current main branch in response to their Issue 2518, and it seems to work fine on the RDFLib side. However, mypy --strict flagged some issues in case_validate on import pyshacl no longer being appropriate to tag with type:ignore, and pyshacl.validate not having an attribute validate. For some reason, I can't trigger these errors in the current develop branch.

The short story seems to be that we should wait until pyshacl issues a release adopting rdflib >=7 before expanding case-utils to using that version.

[ajnelson-nist]

Closed with the merge of #136 .