fix(RDFLib): Remove rdflib-jsonld dependency and use RDFLib v6 jsonld builtin parser
the rdflib-jsonld lib was not compatible with the new setuptools (v.58.0.1 and superior) since there is no "use_2to3" function anymore
BREAKING CHANGE: The RDFLib v6 does not support python 3.6 anymore. With this dependency, CubicWeb neither.