Specialize syntax error for assignment expressions in invalid places

Feature or enhancement

Proposal:

There are a few places where (unparenthesized) assignment expressions are not accepted but users might think of using them. For example:

>>> func(arg=x := foo())
  File "<python-input-0>", line 1
    func(arg=x := foo())
               ^^
SyntaxError: invalid syntax

I find this error pretty confusing. It's not clear why this is a syntax error or what users should do it fix it (i.e. add parentheses).

I propose adding a specialized syntax error to point users in the right direction, similar to what's currently done for assert:

>>> assert x := 1
  File "<python-input-12>", line 1
    assert x := 1
           ^^^^^^
SyntaxError: cannot use named expression without parentheses here

Refs #138716, #146260

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

首先重现 issue 中的示例，并阅读 CPython 对 assignment expressions 的语法错误处理，将其与现有针对 assert 的专用诊断进行比较。当无效的未加括号 assignment expressions 收到明确提示、引导用户使用括号，同时有效形式保留现有行为时，即表示完成。