rdflib 6 compatibility

[tcmitchell]

• Switch to N-Triples 1.1 format to avoid a confusing encoding warning
• Use rdflib.Graph.parse instead of rdflib.Graph.load because load is deprecated in rdflib 6
• Handle bytes or str from rdflib.Graph.serialize
  • rdflib 6 returns str from serialize instead of bytes as rdflib 5 did
• Add a temporary workaround for unsightly user warnings
• Stop importing jsonld parser because it is no longer necessary in rdflib 6
• Change dependencies to use rdflib 6

Closes #279