Start specifying RDFLib version ceilings

This sets `case-utils` and downstream Python tooling to take care in
reviewing new RDFLib releases for breaking changes.  RDFLib Issue 2402
and Discussion 2395 outline an upcoming development period for RDFLib
that may see more frequent SEMVER-major version releases.

References:
* RDFLib/rdflib#2402

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

setup.cfg

@@ -21,7 +21,7 @@ include_package_data = true
 install_requires =
     pandas
     pyshacl
-    rdflib >= 6.2.0
+    rdflib >= 6.2.0, < 7.0.0
     requests
     tabulate
 packages = find: