Make test.test_support.EnvironmentVarGuard behave like a dictionary.

All changes are mirrored to the underlying os.environ dict, but rolled back
on exit from the with block.

Lib/test/test_urllib.py

@@ -103,7 +103,7 @@ class ProxyTests(unittest.TestCase):
         # Delete all proxy related env vars
         for k, v in os.environ.iteritems():
             if 'proxy' in k.lower():
-                env.unset(k)
+                del env[k]
 
     def tearDown(self):
         # Restore all proxy related env vars