(Merge 3.2) Issue #13436: Fix unsetenv() test on Windows

Lib/test/test_os.py

@@ -451,6 +451,7 @@ class EnvironTests(mapping_tests.BasicTestMappingProtocol):
         if sys.platform == "win32":
             # an environment variable is limited to 32,767 characters
             key = 'x' * 50000
+            self.assertRaises(ValueError, os.environ.__delitem__, key)
         else:
             # "=" is not allowed in a variable name
             key = 'key='