[3.9] bpo-41069: Make TESTFN and the CWD for tests containing non-ascii characters. (GH-21035). (GH-21156)

(cherry picked from commit 700cfa8c90a90016638bac13c4efd03786b2b2a0)
diff --git a/Lib/test/test_uu.py b/Lib/test/test_uu.py
index c8709f7..4c639b7 100644
--- a/Lib/test/test_uu.py
+++ b/Lib/test/test_uu.py
@@ -174,8 +174,8 @@
 class UUFileTest(unittest.TestCase):
 
     def setUp(self):
-        self.tmpin  = support.TESTFN + "i"
-        self.tmpout = support.TESTFN + "o"
+        self.tmpin  = support.TESTFN_ASCII + "i"
+        self.tmpout = support.TESTFN_ASCII + "o"
         self.addCleanup(support.unlink, self.tmpin)
         self.addCleanup(support.unlink, self.tmpout)