Updated codegen to work with python3.

Most changes were just replacing usage of "has_key" with "in".
Also updated the sorting function which was simplified and
changed to a "key" function instead of "cmp" (which is no longer
supported in python3. Verified everything builds with
python 2.7 and 3.

https://bugzilla.gnome.org/show_bug.cgi?id=678066

gio/gdbus-2.0/codegen/codegen_main.py

@@ -184,8 +184,8 @@ def codegen_main():
 
     c_code = opts.generate_c_code
     if c_code:
-        h = file(c_code + '.h', 'w')
-        c = file(c_code + '.c', 'w')
+        h = open(c_code + '.h', 'w')
+        c = open(c_code + '.c', 'w')
         gen = codegen.CodeGenerator(all_ifaces,
                                     opts.c_namespace,
                                     opts.interface_prefix,