Adapt code to newer networkx version

[steve725]

Hi,
I'm using networkx 2.6.1 and got two errors in pytopkapi/pretreatment.py.
To fix the errors I had to change:

• change line 194:
  from
  ar_label_sort = np.array(nx.topological_sort(network_dag))
  to
  ar_label_sort = np.array(list(nx.topological_sort(network_dag)))

• change line 201:
  from
  dag_copy = nx.copy.deepcopy(network_dag)
  to
  dag_copy = network_dag.copy()

[JustinPringle]

heavens above! thank you!!