Raise error for parenthesized target in walrus expression

Author: Devansh-b

Grammar/python.gram

@@ -1261,6 +1261,9 @@ invalid_expression:
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "t-string: lambda expressions are not allowed without parentheses") }
 
 invalid_named_expression(memo):
+    | '(' a=NAME ')' ':=' expression {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a, "cannot parenthesize target name in assignment expression") }
     | a=expression ':=' expression {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
             a, "cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }