For anyone who has the same problem, here is a quick bug fix until the problem is finally resolved:
In your graph-tool-2.35 code, replace the file graph_tool/draw/graphviz_draw.py by this one:
http://www.rolf-sander.net/tmp/graphviz_draw.py
FYI: This replacement file graphviz_draw.py is taken from graph-tool version 2.33, and a bug fix from Jeff Trull has already been applied, see:
https://nabble.skewed.de/graphviz-draw-cannot-plot-loops-anymore-td4028466.h...
-- Sent from: https://nabble.skewed.de/