test_contextlib wasn't actually being run by regrtest.py. Or more precisely,

it was being run, but no tests were actually executed!

Lib/test/test_contextlib.py

@@ -2,12 +2,14 @@
 
 from __future__ import with_statement
 
+import sys
 import os
 import decimal
 import tempfile
 import unittest
 import threading
 from contextlib import *  # Tests __all__
+from test.test_support import run_suite
 
 class ContextManagerTestCase(unittest.TestCase):
 
@@ -327,5 +329,11 @@ class DecimalContextTestCase(unittest.TestCase):
             self.fail("Didn't raise ZeroDivisionError")
 
 
+# This is needed to make the test actually run under regrtest.py!
+def test_main():
+    run_suite(
+        unittest.defaultTestLoader.loadTestsFromModule(sys.modules[__name__])
+    )
+
 if __name__ == "__main__":
-    unittest.main()
+    test_main()