Dot parser fails on graph attributes

Bug reports:

On latest docker repeats, repeats under python 2/3. Seems issue inside cpp.

To repeat:

Fails

echo 'digraph Wtf {graph[layout="dot"]}' | python -c 'import sys; import graph_tool.all as gt; g = gt.load_graph(sys.stdin, fmt="dot");'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/graph_tool/__init__.py", line 2936, in load_graph
    g.load(file_name, fmt, ignore_vp, ignore_ep, ignore_gp)
  File "/usr/lib/python2.7/dist-packages/graph_tool/__init__.py", line 2485, in load
    ignore_vp, ignore_ep, ignore_gp)
RuntimeError: boost::bad_any_cast: failed conversion using boost::any_cast

Works

echo 'digraph Wtf {graph[]}' | python -c 'import sys; import graph_tool.all as gt; g = gt.load_graph(sys.stdin, fmt="dot");'