_1password-gui: Fix update-sources.py truncation bug (#422521)

class Sources(OrderedDict):

    def __init__(self):

        self._jsonfp = open("sources.json", "r+")

        self.update(json.load(self._jsonfp))

        self._jsonfp.seek(0, os.SEEK_SET)

        with open("sources.json", "r") as fp:

            self.update(json.load(fp))

    def persist(self):

        json.dump(self, self._jsonfp, indent=2)

        self._jsonfp.write("\n") # keep fmt.check happy

        with open("sources.json", "w") as fp:

            print(json.dumps(self, indent=2), file=fp)

class GPG:

    def __new__(cls):