[BUG] graphviz methods should raise exceptions when passing int64 IDs

[DataBoyTX]

Describe the bug

when IDs are int64, graphviz_layout() throws an error:

ValueError: You are trying to merge on object and int64 columns. If you wish to proceed you should use pd.concat

throw an exception, or potentially cast IDs to strings to get graphviz_layout to complete successfully.

To reproduce:

graphviz_int64_bug.zip