Adjust use of libgvc to fix two graphviz_draw issues

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.
2 jobs for bugfix/graphviz-draw in 113 minutes and 54 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #4866
amd64
job_clang_amd64

01:53:53

passed #4865
amd64
job_gcc_amd64

01:33:33