Temporary - Pin rdflib to pre-6.0.0

This patch establishes the baseline state of unit tests passing.

Unit test results: 5 tests report a FAIL status, all related to JSON-LD
compaction. I suggest that these tests are out of scope of the mission
of restoring builds functioning, related to removing the use_2to3 flag.

References:
* RDFLib/rdflib#1405

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

setup.py

@@ -93,7 +93,7 @@ def md2pypi(filename):
         "Natural Language :: English",
     ],
     test_suite="nose.collector",
-    install_requires=["rdflib>=5.0.0"],
+    install_requires=["rdflib<6.0.0"],
     tests_require=["nose"],
     command_options={
         "build_sphinx": {