bpo-30167: Add test for module.__cached__ is None (GH-7617)

Lib/test/test_site.py

@@ -389,6 +389,17 @@ class ImportSideEffectTests(unittest.TestCase):
                             "expected absolute path, got {}"
                             .format(os__cached__.decode('ascii')))
 
+    def test_abs_paths_cached_None(self):
+        """Test for __cached__ is None.
+
+        Regarding to PEP 3147, __cached__ can be None.
+
+        See also: https://bugs.python.org/issue30167
+        """
+        sys.modules['test'].__cached__ = None
+        site.abs_paths()
+        self.assertIsNone(sys.modules['test'].__cached__)
+
     def test_no_duplicate_paths(self):
         # No duplicate paths should exist in sys.path
         # Handled by removeduppaths()