Use standard method for closing QApp when last window is closed.

setQuitOnLastWindowClosed(True) should be equivalent to
lastWindowClosed.connect(quit), but is more introspectable (e.g.
third-parties can check the value of quitOnLastWindowClosed()).

AFAICT the code used lastWindowClosed.connect(quit) because it dates
back to Qt3, where setQuitOnLastWindowClosed did not exist yet.

Author: anntzer

lib/matplotlib/backends/backend_qt.py

@@ -140,7 +140,7 @@ def _create_qApp():
             image = str(cbook._get_data_path('images/matplotlib.svg'))
             icon = QtGui.QIcon(image)
             app.setWindowIcon(icon)
-        app.lastWindowClosed.connect(app.quit)
+        app.setQuitOnLastWindowClosed(True)
         cbook._setup_new_guiapp()
         if qt_version == 5:
             app.setAttribute(QtCore.Qt.AA_UseHighDpiPixmaps)