[graph-tool] New discourse forum for graph-tool

Dear graph-tool users, There is now an official discourse forum for graph-tool available at: https://forum.skewed.de/c/graph-tool This mailing list is going to remain active — nothing will change, but many people nowadays prefer such web discussions over mailing lists. The link to both the mailing list and forum is now available in the project's webpage. Feel free to use what you prefer. Best, Tiago -- Tiago de Paula Peixoto <tiago@skewed.de>