Allow graph_tool.draw.draw_hierarchy to be used without GTK installed

Status Job ID Name Coverage
  Test
passed #4717
amd64
job_clang_amd64

03:59:16

passed #4709
amd64
job_gcc_amd64

01:01:13

failed #4708
amd64
job_clang_amd64

failed #4710
amd64
job_clang_amd64

01:52:44

failed #4707
amd64
job_gcc_amd64