Add utf8 encoding to json parsing

[KwentiN-ui]

First of all, thank you for this awesome project!
I just came across your NodeGraph().set_context_menu_from_file Method, which is very handy. I have one small issue though.
My application is developed in German, so ÄÖÜ and other Non-ASCII characters don't render correctly in the context menu. I fixed this in my local version of this project by passing

# graph.py:910-911
with file.open(encoding="utf8") as f:
    data = json.load(f)

Before:

After:

To my knowledge, this should not have an effect on other parts of the code.

Thank you for your time!