Discord api websocket gateway connection disconnects after some time

I don't know why but it keeps disconnecting. Sometimes it works for 1 hour, sometimes less and sometimes 2.5 hour-ish

import aiohttp
import asyncio
import json
import datetime, time

token = "123"

payload = {
    'op': 2,
    "d": {
        "token": token,
        "properties": {
            "$os": "windows",
            "$browser": "chrome",
            "$device": 'pc'
        }
    }
}



last_sequence = "null"

async def main():
    global last_sequence


    session = aiohttp.ClientSession()
    async with  session.ws_connect('wss://gateway.discord.gg/?v=9&encording=json') as ws:
        async for msg in ws:
            data = json.loads(msg.data)

            if data["op"] == 10:  # Hello
                await ws.send_json(payload)

                # (Keeps the connection alive!)
                asyncio.ensure_future(heartbeat(ws, data['d']['heartbeat_interval']))
            elif data["op"] == 11:  # Heartbeat ACK
                #print("Heartbeat Received")
                pass

            elif data["op"] == 0:  # Dispatch
                try:
                    if data['d']['guild_id']==("669653521007902751") and data['d']['channel_id']==("669653521007902766"):
                        print(f"{data['d']['content']}")
                        last_sequence = data['s']   
                        #print(data)  

                except:
                    pass

            elif data["op"] == 3: # Presence Update 
                print("This is OP 3", data)
            elif data["op"] == 4: # Voice State Update  
                print("This is OP 4", data)
            elif data["op"] == 6: # Resume
                print("This is OP 6", data)
            elif data["op"] == 7: # Reconnect, i don't know how this works. I was just testing it.
                await ws.send_json(
                    {"op": 6,
                        "d": {
                            "token":token,
                            "session_id": "null",
                            "seq": last_sequence
                    }})
                print("This is OP 7", data)
            elif data["op"] == 8: # Request Guild Members   
                print("This is OP 8", data)
            elif data["op"] == 9: # Invalid Session
                print("This is OP 9", data)


            else:
                print("What happened?", data)
                
        #await session.close()
    



async def heartbeat(ws, interval):
    while True:
        await asyncio.sleep(interval / 1000)  # seconds
        await ws.send_json({
            "op": 1,  # Heartbeat
            "d": last_sequence
        })
        print("Heartbeat Sent", last_sequence, time.strftime("%H:%M:%S", time.localtime()))




loop = asyncio.get_event_loop()
loop.run_until_complete(main())
loop.close()

The error is

This is OP 7 {'t': None, 's': None, 'op': 7, 'd': None} Task was destroyed but it is pending! task: <Task pending name='Task-4' coro=<heartbeat() running at C:\Users\test\Desktop\Discord_Test\aio_test.py:83> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x00000214675B83D0>()]>>