[3.11] Fix SyntaxWarning in test_syntax.py (GH-112944) (GH-112956)

(cherry picked from commit 97cd45b)

Co-authored-by: Anthony Sottile <asottile@umich.edu>

Author: miss-islington

Lib/test/test_syntax.py

@@ -2145,7 +2145,7 @@ def test_error_parenthesis(self):
 
         # Examples with dencodings
         s = b'# coding=latin\n(aaaaaaaaaaaaaaaaa\naaaaaaaaaaa\xb5'
-        self._check_error(s, "'\(' was never closed")
+        self._check_error(s, r"'\(' was never closed")
 
     def test_error_string_literal(self):