PyQt: PyUIC-generated keyboard shortcuts not working under Linux (Ubuntu 16.10)

I want to create a widget-based PyQt5 application. I use Qt Designer to make UI forms for it. In this application, I have a few keyboard shortcuts for menu actions. The shortcuts work under Windows, but do nothing under Ubuntu (but still work if explicitly clicked by mouse).

Here' the code generated by PyUIC with my code after it. You should press Ctrl+C to close the window, but alas, the window is closed only if explicitly click to this menu action.

Code:

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# -*- coding: utf-8 -*-
 
# Form implementation generated from reading ui file ui_demo_window.ui
#
# Created by: PyQt5 UI code generator 5.8.2
#
# WARNING! All changes made in this file will be lost!
 
from PyQt5 import QtCore, QtGui, QtWidgets
 
class Ui_DemoWindow(object):
    def setupUi(self, DemoWindow):
        DemoWindow.setObjectName("DemoWindow")
        DemoWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(DemoWindow)
        self.centralwidget.setObjectName("centralwidget")
        DemoWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(DemoWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 25))
        self.menubar.setObjectName("menubar")
        self.menuFile = QtWidgets.QMenu(self.menubar)
        self.menuFile.setObjectName("menuFile")
        DemoWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(DemoWindow)
        self.statusbar.setObjectName("statusbar")
        DemoWindow.setStatusBar(self.statusbar)
        self.close_action = QtWidgets.QAction(DemoWindow)
        self.close_action.setObjectName("close_action")
        self.menuFile.addAction(self.close_action)
        self.menubar.addAction(self.menuFile.menuAction())
 
        self.retranslateUi(DemoWindow)
        self.close_action.triggered.connect(DemoWindow.close)
        QtCore.QMetaObject.connectSlotsByName(DemoWindow)
 
    def retranslateUi(self, DemoWindow):
        _translate = QtCore.QCoreApplication.translate
        DemoWindow.setWindowTitle(_translate("DemoWindow", "MainWindow"))
        self.menuFile.setTitle(_translate("DemoWindow", "File"))
        self.close_action.setText(_translate("DemoWindow", "Close"))
        self.close_action.setShortcut(_translate("DemoWindow", "Ctrl+C"))
 
 
 
### AND HERE'S MY CODE
 
 
import sys
 
from PyQt5 import QtWidgets, QtGui, QtCore
 
class DemoWindow(QtWidgets.QMainWindow, Ui_DemoWindow):
    def __init__(self, parent=None):
        QtWidgets.QMainWindow.__init__(self, parent)
        self.setupUi(self)
 
 
if __name__ == "__main__":
 
    #########################################
    sys._excepthook = sys.excepthook
    def exception_hook(exctype, value, traceback):
        sys._excepthook(exctype, value, traceback)
        sys.exit(1)
    sys.excepthook = exception_hook
    ##########################################
 
 
    app = QtWidgets.QApplication(sys.argv)
 
    window = DemoWindow()
    window.show()
 
    sys.exit(app.exec_())

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Can anyone tell me, what can I do to fix it?

P.S. If needed, here's my UI form code, generated by Qt Designer:

Code:

---

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>DemoWindow</class>
 <widget class="QMainWindow" name="DemoWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget"/>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>800</width>
     <height>25</height>
    </rect>
   </property>
   <widget class="QMenu" name="menuFile">
    <property name="title">
     <string>File</string>
    </property>
    <addaction name="close_action"/>
   </widget>
   <addaction name="menuFile"/>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
  <action name="close_action">
   <property name="text">
    <string>Close</string>
   </property>
   <property name="shortcut">
    <string>Ctrl+C</string>
   </property>
  </action>
 </widget>
 <resources/>
 <connections>
  <connection>
   <sender>close_action</sender>
   <signal>triggered()</signal>
   <receiver>DemoWindow</receiver>
   <slot>close()</slot>
   <hints>
    <hint type="sourcelabel">
     <x>-1</x>
     <y>-1</y>
    </hint>
    <hint type="destinationlabel">
     <x>399</x>
     <y>299</y>
    </hint>
   </hints>
  </connection>
 </connections>
</ui>

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