Add .py extension if it is missing from keymap file.

This is for bug #28141
While not really a bug, it makes it a lot easyer to use if it 
has the exension.  (Isn't hidden from the user when they try to load it...)

Kent

release/scripts/startup/bl_ui/space_userpref_keymap.py

@@ -592,6 +592,9 @@ class WM_OT_keyconfig_export(bpy.types.Operator):
         if not self.filepath:
             raise Exception("Filepath not set")
 
+        if not self.filepath.endswith('.py'):
+            self.filepath += '.py'
+
         f = open(self.filepath, "w")
         if not f:
             raise Exception("Could not open file")