In the file backend_qt5.py in line 126 a new QApplication object is created. However, the passed argument is a list with a binary string. This causes PySide2 to print ugly

QSettings::value: Empty key passed
QSettings::value: Empty key passed

Correctly, the string should be passed, which will make the warning go away.

To resolve this issue, please replace

qApp = QtWidgets.QApplication([b"matplotlib"])

to this

qApp = QtWidgets.QApplication(["matplotlib"])