MIGRATION: Python 3

[janjoswig]

This makes the package compatible with Python 3. I changed the following:

• Use of dict.items() instead of dict.iteritems() (same for keys and values)
• Subgraphs are now not "freezed" anymore when using a more recent version of networkx (solves issue NetworkXError: SubGraph Views are readonly. Mutations not allowed #67 )
• Explicite use of "latin1" encoding when unpickling with load_tree
• Use of assertCountEqual instead of assertItemsEqual in the tests
• Use of list() to exhaust map generator and to convert dict.values to NumPy array

All tests pass (with Python 3.8) and the quickstart example from the documentation works as expected.