Fix interrupting GTK on plain Python

When the application is shut down, it's intended that all windows are
closed. This shutdown event is also run if interrupted with Ctrl-C.
But then the main loop is no longer running, and windows do not notice
the close event sent to them.

In IPython, the input hook runs the loop and allows the windows to close
down. But in plain Python, there is no input hook integration, and the
windows sit around in limbo without accepting any events.

Absent any new input hook integrations, the workaround is to iterate the
main loop on interrupt.

Author: QuLogic

lib/matplotlib/backends/_backend_gtk.py

@@ -169,6 +169,13 @@ def mainloop():
 
         try:
             _application.run()  # Quits when all added windows close.
+        except KeyboardInterrupt:
+            # Ensure all windows can process their close event from
+            # _shutdown_application.
+            context = GLib.MainContext.default()
+            while context.pending():
+                context.iteration(True)
+            raise
         finally:
             # Running after quit is undefined, so create a new one next time.
             _application = None