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

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 9eb7ebe..7ffe01d 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -1055,8 +1055,9 @@
             file_path = os.path.join(cwd, 't.py')
             with open(file_path, 'w') as f:
                 f.write('f"{a b}"') # This generates a SyntaxError
-            _, _, stderr = assert_python_failure(file_path)
-        self.assertIn(file_path, stderr.decode('utf-8'))
+            _, _, stderr = assert_python_failure(file_path,
+                                                 PYTHONIOENCODING='ascii')
+        self.assertIn(file_path.encode('ascii', 'backslashreplace'), stderr)
 
     def test_loop(self):
         for i in range(1000):