Fix test_site from modifying sysconfig._CONFIG_VARS.

Lib/test/test_site.py

@@ -39,6 +39,7 @@ class HelperFunctionsTests(unittest.TestCase):
         self.old_base = site.USER_BASE
         self.old_site = site.USER_SITE
         self.old_prefixes = site.PREFIXES
+        self.original_vars = sysconfig._CONFIG_VARS
         self.old_vars = copy(sysconfig._CONFIG_VARS)
 
     def tearDown(self):
@@ -47,7 +48,9 @@ class HelperFunctionsTests(unittest.TestCase):
         site.USER_BASE = self.old_base
         site.USER_SITE = self.old_site
         site.PREFIXES = self.old_prefixes
-        sysconfig._CONFIG_VARS = self.old_vars
+        sysconfig._CONFIG_VARS = self.original_vars
+        sysconfig._CONFIG_VARS.clear()
+        sysconfig._CONFIG_VARS.update(self.old_vars)
 
     def test_makepath(self):
         # Test makepath() have an absolute path for its first return value