Serialization of graph fails if there are accepted_connection_types/rejected_connection_types defined

[claudio-alvaro-wbgames]

Python's json doesn't serialize set objects. I unfortunately can't submit a PR but the fix is relatively simple, in case anyone else encounters this issue.

line 553 of NodeGraphQt/base/model.py:

        if accept_ptype not in connection_data:
            connection_data[accept_ptype] = [accept_pname]
        else:
            # ensure data remains a set instead of list after json de-serialize
            connection_data[accept_ptype] = list(set(connection_data[accept_ptype]) | {accept_pname})

[jchanvfx]

@claudio-alvaro-wbgames Thanks for pointing out this issue I'll make some tweaks.