Catch more errors inside fstring_replacement_field

Author: sunmy2019

Grammar/python.gram

@@ -1358,10 +1358,7 @@ invalid_kvpair:
 invalid_starred_expression:
     | a='*' expression '=' b=expression { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "cannot assign to iterable argument unpacking") }
 invalid_replacement_field:
-    | '{' a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: expression required before '='") }
-    | '{' a=':' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: expression required before ':'") }
-    | '{' a='!' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: expression required before '!'") }
-    | '{' a='}' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: empty expression not allowed") }
+    | '{' !(yield_expr | star_expressions) { RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("f-string: expecting expression after '{'")}
     | '{' (yield_expr | star_expressions) !('=' | '!' | ':' | '}') {
         PyErr_Occurred() ? NULL : RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("f-string: expecting '=', or '!', or ':', or '}'") }
     | '{' (yield_expr | star_expressions) '=' !('!' | ':' | '}') {