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

@@ -1,34 +1,34 b''
 # encoding: utf-8
 """
 Enable Gtk3 to be used interacive by IPython.
 
 Authors: Thomi Richards
 """
 #-----------------------------------------------------------------------------
 # Copyright (c) 2012, the IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 import sys
 from gi.repository import Gtk, GLib
 
 #-----------------------------------------------------------------------------
 # Code
 #-----------------------------------------------------------------------------
 
 def _main_quit(*args, **kwargs):
     Gtk.main_quit()
     return False
 
 
 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

@@ -1,12 +1,12 b''
 """prompt_toolkit input hook for GTK 3
 """
 
 from gi.repository import Gtk, GLib
 
 def _main_quit(*args, **kwargs):
     Gtk.main_quit()
     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()