fixup in QSpinBox does not work

from PySide6.QtGui import QValidator
from PySide6.QtWidgets import QApplication, QSpinBox

class MySpinBox(QSpinBox):
    def __init__(self, parent=None):
        super().__init__(parent)

    def stepBy(self, steps):
        self.setValue(self.value() + 5 * steps)

    def validate(self, input, pos):
        if not input:
            return QValidator.State.Intermediate

        try:
            number = int(input)
        except ValueError:
            return QValidator.State.Invalid

        if number % 5 == 0:
            return QValidator.State.Acceptable
        else:
            return QValidator.State.Intermediate

    def fixup(self, input):
        number = int(input)
        # Round up to next multiple of 5.
        number += 5 - number % 5
        return str(number)

if __name__ == "__main__":
    app = QApplication()
    spinbox = MySpinBox()
    spinbox.show()
    app.exec()