This fixes issue #687

The only observable effect should be that calling with output=None and (output_format="xlib" or fork=True) no longer causes an AttributeError.

The effect of setting "output" to None should be more clear now.

One issue was reported to the mailing list on 2020-08-17 by Rolf
Sander; the testcase is:

import graph_tool.all as gt
g = gt.Graph()
v0 = g.add_vertex()
v1 = g.add_vertex()
e01 = g.add_edge(v0,v1)
e11 = g.add_edge(v1,v1) # loop
gt.graphviz_draw(g)

The other is issue #597 "Graphviz_draw core dumping when using HTML labels"

Basically these changes add extra type information for interacting
with libgvc.

This improves performance of the Python API.

This fixes issue #386

This resolves ticket #100