bpo-40246: Revert reporting of invalid string prefixes (GH-19888)

Due to backwards compatibility concerns regarding keywords immediately followed by a string without whitespace between them (like in `bg="#d00" if clear else"#fca"`) will fail to parse,
commit 41d5b94af44e34ac05d4cd57460ed104ccf96628 has to be reverted.
diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index fe465b7..ac5aa9a 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -864,7 +864,7 @@
                              "Bf''",
                              "BF''",]
         double_quote_cases = [case.replace("'", '"') for case in single_quote_cases]
-        self.assertAllRaise(SyntaxError, 'invalid string prefix',
+        self.assertAllRaise(SyntaxError, 'unexpected EOF while parsing',
                             single_quote_cases + double_quote_cases)
 
     def test_leading_trailing_spaces(self):