Make gdb pretty-printers compatible with Python3

On some systems gdb is linked against python3 where "long" no longer
exists. In this case should be using int.

https://bugzilla.gnome.org/show_bug.cgi?id=720635

glib/glib.py

@@ -1,4 +1,8 @@
 import gdb
+import sys
+
+if sys.version_info[0] >= 3:
+    long = int
 
 # This is not quite right, as local vars may override symname
 def read_global_var (symname):

gobject/gobject.py

@@ -1,6 +1,11 @@
 import os.path
 import gdb
 import glib
+import sys
+
+if sys.version_info[0] >= 3:
+    long = int
+
 try:
     import gdb.backtrace
     import gdb.command.backtrace