choose correct import machinery in unit test

Fixes https://pagure.io/koji/issue/4306

tests/test_lib/test_plugin.py

@@ -9,17 +9,14 @@ from six.moves import range
 import sys
 import unittest
 
-try:
-    import imp
-    importlib = None
-except ImportError:
-    import importlib
-    imp = None
-
 import koji
 import koji.util
 import koji.plugin
 
+# check which import machinery the lib is using
+imp = getattr(koji.plugin, 'imp', None)
+importlib = koji.plugin.importlib
+
 
 class TestCallbackDecorators(unittest.TestCase):