Fix SyntaxWarning in test_syntax.py (pythonGH-112944)

Author: asottile

Lib/test/test_syntax.py

@@ -2336,7 +2336,7 @@ 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):