Use test.test_support.EnvironmentVarGuard where tests change environment vars.
diff --git a/Lib/test/test_site.py b/Lib/test/test_site.py
index 51e36da..03f6d80 100644
--- a/Lib/test/test_site.py
+++ b/Lib/test/test_site.py
@@ -5,7 +5,7 @@
 
 """
 import unittest
-from test.test_support import run_unittest, TESTFN
+from test.test_support import run_unittest, TESTFN, EnvironmentVarGuard
 import __builtin__
 import os
 import sys