This line fails with rdflib 6.1.1 because the OWL namespace defined in rdflib does not support the `OWL.Datatype` property: https://github.com/RDFLib/OWL-RL/blob/a77e1791b88b54aace609bc6000aac14c7add4ff/owlrl/OWLRLExtras.py#L354