Deprecation warnings for ConjunctiveGraph

[cmeesters]

Hi,

In an attempt to fix the CI (see PR #207), I encountered

tests/test_nanopub.py: 4 warnings
  /home/cm/miniforge3/envs/nanopub/lib/python3.13/site-packages/rdflib/plugins/parsers/trig.py:155: DeprecationWarning: ConjunctiveGraph is deprecated, use Dataset instead.
    conj_graph = ConjunctiveGraph(store=graph.store, identifier=graph.identifier)

tests/test_fdo_op_validate_integration.py::test_validate_fdo_records[https://w3id.org/np/RAsSeIyT03LnZt3QvtwUqIHSCJHWW1YeLkyu66Lg4FeBk/nanodash-readme]

and

tests/test_nanopub.py::test_specific_file
  /home/cm/miniforge3/envs/nanopub/lib/python3.13/site-packages/rdflib/plugins/parsers/jsonld.py:168: DeprecationWarning: ConjunctiveGraph is deprecated, use Dataset instead.
    conj_sink = ConjunctiveGraph(store=sink.store, identifier=sink.identifier)

Fixing this will not be part of this particular PR. Perhaps it deserves attention, though.

Cheers
Christian