Missing 'pydot' dependency fails on networkx nx_pydot

[rakhimov]

Traceback (most recent call last):
  File "/.../python3env/bin/cppdep", line 11, in <module>
    sys.exit(main())
  File "/.../python3env/lib/python3.5/site-packages/cppdep/__main__.py", line 59, in main
    analysis.analyze(printer, args)
  File "/.../python3env/lib/python3.5/site-packages/cppdep/cppdep.py", line 797, in analyze
    lambda x: isinstance(x, PackageGroup)))
  File "/.../python3env/lib/python3.5/site-packages/cppdep/cppdep.py", line 779, in _analyze
    digraph.write_dot(graph_name)
  File "/.../python3env/lib/python3.5/site-packages/cppdep/graph.py", line 273, in write_dot
    write_dot(self.digraph, file_basename + '.dot')
  File "<decorator-gen-416>", line 2, in write_dot
  File "/.../python3env/lib/python3.5/site-packages/networkx/utils/decorators.py", line 224, in _open_file
    result = func(*new_args, **kwargs)
  File "/.../python3env/lib/python3.5/site-packages/networkx/drawing/nx_pydot.py", line 54, in write_dot
    P = to_pydot(G)
  File "/.../python3env/lib/python3.5/site-packages/networkx/drawing/nx_pydot.py", line 199, in to_pydot
    pydot = _import_pydot()
  File "/.../python3env/lib/python3.5/site-packages/networkx/drawing/nx_pydot.py", line 348, in _import_pydot
    import pydot