[3.12] gh-105915: Add 'r' prefix to not emit SyntaxWarning in test_fstring (GH-105940) (#105942)

(cherry picked from commit 4b431d2)

Co-authored-by: @sunmy2019
Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Author: miss-islington

Lib/test/test_fstring.py

@@ -773,7 +773,7 @@ def __format__(self, format_spec):
         self.assertEqual(f'{CustomFormat():\n}', '\n')
         self.assertEqual(f'{CustomFormat():\u2603}', '☃')
         with self.assertWarns(SyntaxWarning):
-            exec('f"{F():¯\_(ツ)_/¯}"', {'F': CustomFormat})
+            exec(r'f"{F():¯\_(ツ)_/¯}"', {'F': CustomFormat})
 
     def test_side_effect_order(self):
         class X: