bpo-36160: Fix test_site so that it can run independently of other tests (GH-12131) (GH-26263)

(cherry picked from commit 1270ad6)

Co-authored-by: native-api <[email protected]>

Author: miss-islington

Lib/test/test_site.py

@@ -76,8 +76,10 @@ def tearDown(self):
         site.USER_SITE = self.old_site
         site.PREFIXES = self.old_prefixes
         sysconfig._CONFIG_VARS = self.original_vars
-        sysconfig._CONFIG_VARS.clear()
-        sysconfig._CONFIG_VARS.update(self.old_vars)
+        # _CONFIG_VARS is None before get_config_vars() is called
+        if sysconfig._CONFIG_VARS is not None:
+            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