Use importlib not imp

The imp module is gone from Python 3.12, but importlib has
the same reload() function. Let's use it.

Change-Id: Iab4381e743a443285a86941de195189e26e9e0fc

os_ken/tests/unit/cmd/test_manager.py

@@ -17,7 +17,7 @@
 import unittest
 from unittest import mock
 
-from imp import reload
+from importlib import reload
 
 from os_ken.cmd.manager import main
 

os_ken/tests/unit/ofproto/test_ofproto.py

@@ -16,7 +16,7 @@
 
 # vim: tabstop=4 shiftwidth=4 softtabstop=4
 
-from imp import reload
+from importlib import reload
 
 import unittest
 import logging