How to close a task created with asyncqt.QEventLoop in PyQt5

Question

I want to use PyQt5 to access flows in mitmproxy. Since mitmproxy is asynchronous, I use asyncqt.QEventLoop to create a loop to share with mitmproxy. I can start the mitmproxy master service now, but I am unable to stop the master service. How can I correctly stop this task?

import asyncio
from PyQt5 import QtWidgets
from asyncqt import QEventLoop
from mitmproxy import addons
from mitmproxy.options import Options
from mitmproxy.master import Master

class MitmProxy:
    def __init__(self, options: Options) -> None:
        self.loop = asyncio.get_event_loop()
        print(id(loop), loop)
        self.master = Master(options, event_loop=self.loop)
        self.master.addons.add(*addons.default_addons())

    def run(self) -> None:
        self.task = self.loop.create_task(self.master.run())

    def stop(self) -> None:
        self.task.cancel()
        if self.task.cancelled():
            print("task cancelled, shutdown")
            self.master.shutdown()
        print('task cancel over')

class MyWidget(QtWidgets.QWidget):
    def __init__(self):
        super().__init__()
        self.loop = asyncio.get_event_loop()
        self.mitm = None

        self.lyt = QtWidgets.QHBoxLayout(self)
        self.btn_start = QtWidgets.QPushButton("Start")
        self.btn_stop = QtWidgets.QPushButton("Stop")

        self.btn_start.clicked.connect(self.add_task)
        self.btn_stop.clicked.connect(self.stop_task)

        self.lyt.addWidget(self.btn_start)
        self.lyt.addWidget(self.btn_stop)

    def add_task(self):
        self.mitm = MitmProxy(
            Options(listen_host="0.0.0.0", listen_port=8080, http2=True)
        )
        self.mitm.run()

    def stop_task(self):
        self.mitm.stop()


if __name__ == "__main__":
    app = QtWidgets.QApplication([])
    loop = QEventLoop(app)
    print(id(loop), loop)
    asyncio.set_event_loop(loop)

    label = MyWidget()
    label.show()

    with loop:
        loop.run_forever()