Fix show_error inheritance issue

gui/qt/installwizard.py

@@ -54,8 +54,8 @@ class CosignWidget(QWidget):
         qp.end()
 
 
-# WizardBase must come first as we override show_error
-class InstallWizard(WizardBase, WindowModalDialog):
+# WindowModalDialog must come first as it overrides show_error
+class InstallWizard(WindowModalDialog, WizardBase):
 
     def __init__(self, config, app, plugins):
         title = 'Electrum  -  ' + _('Install Wizard')

gui/qt/util.py

@@ -155,7 +155,7 @@ class CancelButton(QPushButton):
         QPushButton.__init__(self, label or _("Cancel"))
         self.clicked.connect(dialog.reject)
 
-class MessageBoxMixin:
+class MessageBoxMixin(object):
     def question(self, msg, parent=None, title=None, icon=None):
         Yes, No = QMessageBox.Yes, QMessageBox.No
         return self.msg_box(icon or QMessageBox.Question,