Skip test_site if USER_SITE cannot be created

Issue #17758: Skip test_site if site.USER_SITE directory doesn't exist and
cannot be created.

Lib/test/test_site.py

@@ -28,8 +28,13 @@ import site
 
 if site.ENABLE_USER_SITE and not os.path.isdir(site.USER_SITE):
     # need to add user site directory for tests
-    os.makedirs(site.USER_SITE)
-    site.addsitedir(site.USER_SITE)
+    try:
+        os.makedirs(site.USER_SITE)
+        site.addsitedir(site.USER_SITE)
+    except PermissionError as exc:
+        raise unittest.SkipTest('unable to create user site directory (%r): %s'
+                                % (site.USER_SITE, exc))
+
 
 class HelperFunctionsTests(unittest.TestCase):
     """Tests for helper functions.