bpo-42955: Add sys.modules_names (GH-24238)

Add sys.module_names, containing the list of the standard library
module names.
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 3af5b11..729b866 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -986,6 +986,11 @@ def test_orig_argv(self):
         self.assertEqual(proc.stdout.rstrip().splitlines(), expected,
                          proc)
 
+    def test_module_names(self):
+        self.assertIsInstance(sys.module_names, frozenset)
+        for name in sys.module_names:
+            self.assertIsInstance(name, str)
+
 
 @test.support.cpython_only
 class UnraisableHookTest(unittest.TestCase):