Merge branch 'bugfix/graphviz-read-graph-properties' into 'master'

Fix dot graph reader

Closes #450

See merge request count0/graph-tool!35
2 jobs for master in 117 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #4874
amd64
job_clang_amd64

01:57:38

passed #4873
amd64
job_gcc_amd64

01:38:07