rez-context -g with prune-package fails

[nerdvegas]

Appears to be due to an incompatibility with pygraph/pydot. We're already on latest pygraph however (1.8.2), which still has the issue: https://github.com/Shoobx/python-graph

Stacktrace:
File "/sw/install/global/linux_x86_64_CentOS-7/opensource/rez/2.52.1/payload/lib/python2.7/site-packages/rez/cli/_main.py", line 144, in run_cmd
return opts.func(opts, opts.parser, extra_arg_groups)
File "/sw/install/global/linux_x86_64_CentOS-7/opensource/rez/2.52.1/payload/lib/python2.7/site-packages/rez/cli/context.py", line 153, in command
gstr = prune_graph(gstr, req.name)
File "/sw/install/global/linux_x86_64_CentOS-7/opensource/rez/2.52.1/payload/lib/python2.7/site-packages/rez/utils/graph_utils.py", line 171, in prune_graph
g = read_dot(graph_str)
File "/sw/install/global/linux_x86_64_CentOS-7/opensource/rez/2.52.1/payload/lib/python2.7/site-packages/rez/vendor/pygraph/readwrite/dot.py", line 61, in read
if (dotG.get_type() == "graph"):
AttributeError: 'list' object has no attribute 'get_type'