Try calling sip.delete on the test QApplication.

Maybe this helps with the unit tests on Windows?

qutebrowser/test/__init__.py

@@ -20,10 +20,12 @@
 """The qutebrowser test suite."""
 
 import atexit
+import functools
 
+import sip
 from PyQt5.QtWidgets import QApplication
 
 # We create a singleton QApplication here.
 
 qApp = QApplication([])
-atexit.register(qApp.quit)
+atexit.register(functools.partial(sip.delete, qApp))