I think the problem is due to the fact that, in your file, classes are declared as "rdfs:Class" and not "owl:Class". But Owlready expect OWL class.
Same problem occurs for Properties.
I considered the addition of aliases (= considering rdfs:Class as an alias for owl:Class). However, this cannot be done for Properties, since OWL only has ObjectProperty and DataProperty, but no "general" Property.
Basically, your file is an RDFS file but not a "real" OWL ontology. This is why Owlready cannot read it.
MCF, LIMICS, Université Paris 13
I guess I didnt read the description properly. I thought Owlready2 could also read RDFS. However, doesnt really matter as Erlangen CRM is an OWL version of CIDOC anyway and it is working fine with Owlready2.