bpo-40246: Report a better error message for invalid string prefixes (GH-19476)

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 4966392..ef0ccb8 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -841,7 +841,7 @@
         self.assertEqual(f'{f"{y}"*3}', '555')
 
     def test_invalid_string_prefixes(self):
-        self.assertAllRaise(SyntaxError, 'unexpected EOF while parsing',
+        self.assertAllRaise(SyntaxError, 'invalid string prefix',
                             ["fu''",
                              "uf''",
                              "Fu''",