gh-150633: Minor improvement of a newly added test (GH-151103)

Minor improvement of a newly added test.
(cherry picked from commit fccf67a)

Co-authored-by: Barry Warsaw <barry@python.org>

Author: warsaw

Lib/test/test_import/__init__.py

@@ -367,11 +367,11 @@ def test_import_raises_ModuleNotFoundError(self):
     def test_import_null_byte_in_name_raises_ModuleNotFoundError(self):
         # gh-150633: module names containing null bytes should not
         # lead to duplicates in sys.modules
-        before = set(sys.modules.keys())
+        before = set(sys.modules)
         with self.assertRaises(ModuleNotFoundError):
             __import__('zipimport\x00junk')
 
-        self.assertEqual(set(sys.modules.keys()), before)
+        self.assertEqual(set(sys.modules), before)
 
     def test_from_import_missing_module_raises_ModuleNotFoundError(self):
         with self.assertRaises(ModuleNotFoundError):