Log Message:
py-dot: updated to 1.2.4

1.2.4:
- ENH: propagate LD_LIBRARY_PATH when calling GraphViz
- API: raise OSError when a GraphViz executable is not found
- API: add method __str__ to classes Node, Edge, Graph
- API: add arg encoding to Dot methods create, write
- API: quote cluster names when necessary
- API: give source and destination nodes as separate args to Edge.__init__
- API: never ignore src, dst, overwrite if points defined in obj_dict