Chore: Update to rdflib>=6, lower versions are failing the tests

With rdflib-5.x, `testDefaultGraph` produces:

  AttributeError: property 'name' of 'Param' object has no setter

Author: amotl

setup.py

@@ -44,7 +44,7 @@
     ],
     install_requires=[
         "alembic>=0.8.8",
-        "rdflib>=4.0",
+        "rdflib>=6,<8",
         "six>=1.10.0",
         "SQLAlchemy>=1.1.4,<2.0.0",
     ],