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By default, open("test.json") will open the file in text mode, decoding from the encoding determined by your environment. See <https://docs.python.org/3.4/library/functions.html#open>, in particular, "The default encoding is platform dependent (whatever locale.getpreferredencoding() returns)".
If your file’s encoding is determined some other way, you should specify that other encoding. For JSON, I understand UTF-8 is generally always used, so you should call open("test.json", encoding="utf-8").