Update string type from graph.serialize

The type has changed from bytes() to str() since the isomorphic diff
sample file was originally written.

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

Author: ajnelson-nist

tests/src/isomorphic_diff.py

@@ -79,9 +79,9 @@ def _report(
         https://rdflib.readthedocs.io/en/stable/apidocs/rdflib.html#module-rdflib.compare
         """
         for line in sorted(graph.serialize(format="nt").splitlines()):
-            if line.strip() == b"":
+            if line.strip() == "":
                 continue
-            _logger.debug("%s %s", diff_symbol, line.decode("ascii"))
+            _logger.debug("%s %s", diff_symbol, line)
 
     #_report("1", g1)
     #_report("2", g2)