bpo-10049: Add a "no-op" (null) context manager to contextlib (GH-4464)

Adds a simpler and faster alternative to ExitStack for handling
single optional context managers without having to change the
lexical structure of your code.

Lib/test/test_contextlib.py

@@ -252,6 +252,16 @@ class ClosingTestCase(unittest.TestCase):
                 1 / 0
         self.assertEqual(state, [1])
 
+
+class NullcontextTestCase(unittest.TestCase):
+    def test_nullcontext(self):
+        class C:
+            pass
+        c = C()
+        with nullcontext(c) as c_in:
+            self.assertIs(c_in, c)
+
+
 class FileContextTestCase(unittest.TestCase):
 
     def testWithOpen(self):