PyQt6: SystemError in QWidget.setWindowFlags with FramelessWindowHint

[Ales Erjavec]

Hi,

The following example raises a SystemError in setWindowFlags call:
```
from PyQt6.QtCore import Qt
from PyQt6.QtWidgets import QApplication, QWidget

app = QApplication([])

w = QWidget()
f = w.windowFlags()
f = f | Qt.WindowType.FramelessWindowHint
w.setWindowFlags(f)  # << error
```

Windows 10
Python: 3.8.8
PyQt6: 6.1.0.dev2104211125

Best wishes
Aleš Erjavec