nxpd to_pydot doesn't work with networkx 2.4

[mtreinish]

The to_pydot method was using a deprecated function: https://github.com/chebee7i/nxpd/blob/master/nxpd/nx_pydot.py#L249 which has been removed in the latest release 2.4.0: https://networkx.github.io/documentation/stable/release/release_2.4.html#deprecations