Avoid PyGIDeprecationWarning when using Gtk3 backend.

Since pygobject 3.7.3, which was released ~5 years ago, the function
`GLib.io_add_watch` has preferred a call signature other than the one that our
input backends were using.

IPython/lib/inputhookgtk3.py

@@ -29,6 +29,6 b' def _main_quit(*args, **kwargs):'
 
 
 def inputhook_gtk3():
-    GLib.io_add_watch(sys.stdin, GLib.IO_IN, _main_quit)
+    GLib.io_add_watch(sys.stdin, GLib.PRIORITY_DEFAULT, GLib.IO_IN, _main_quit)
     Gtk.main()
     return 0

IPython/terminal/pt_inputhooks/gtk3.py

@@ -8,5 +8,5 b' def _main_quit(*args, **kwargs):'
     return False
 
 def inputhook(context):
-    GLib.io_add_watch(context.fileno(), GLib.IO_IN, _main_quit)
+    GLib.io_add_watch(context.fileno(), GLib.PRIORITY_DEFAULT, GLib.IO_IN, _main_quit)
     Gtk.main()