RXT_FILE and "graph"

[mottosso]

The graph string stored in the RXT file includes invalid string literals, resulting in rez context -g not working.

bleeding-rez/src/rez/utils/graph_utils.py

Line 106 in 3933081

doc = dict(nodes=d_nodes.items(), edges=d_edges.items())

These produce the string:

"{'nodes': dict_items([((('fillcolor', '#FFFF ...

Which should be:

"{'nodes': [((('fillcolor', '#FFFF ...

This only applies to Python 3, however if Rez was launched from Python 3, into a context with Rez and Python 2, then the RXT file would not be compatible.