code: def test_core_number_self_loop(self):
      G = nx.cycle_graph(3)
      G.add_edge(0, 0)
      with pytest.raises(
          nx.NetworkXNotImplemented, match="Input graph has self loops"
      ):
          nx.core_number(G)
  
imports: import pytest
  import networkx
  from networkx.utils import nodes_equal
label_desc: Use image node_id 6 for calling the TestCore obj's underlying member method code with example usage: obj.test_core_number_self_loop() without return types