Improve error message for return in metaclass __new__()

Feature or enhancement

Proposal:

Case1: When overriding new in a custom Meta class and forgetting to return a class object (e.g. by omitting return super().new(..)) Python raises the following error:
TypeError: 'NoneType' object is not callable

Case2: When returning something else other than class object it raises following error:
TypeError: 'str' object is not callable

This messages, while technically correct, lacks guidance for common scenarios and can confuse developers - especially those new to Meta class.

Steps to Reproduce:

Case1:
class NewMeta(type):
def new(cls, name, bases, dct):
print ("Forgot to return!")

class MyClass(metaclass = NewMeta):
def greet(self):
pass

m = MyClass()

Output: TypeError: 'NoneType' object is not callable

Case2:
class NewMeta(type):
def new(cls, name, bases, dct):
return ("Return non-callable object")

class MyClass(metaclass = NewMeta):
def greet(self):
pass

m = MyClass()

Output: TypeError: 'str' object is not callable

Suggested Improvement:
Improve the error message to something like:
TypeError: metaclass new must return a class object. Recommendation: super().new(metacls, name, bases, namespace)?

This small change could save significant debugging time and make advanced features like metaclasses more approachable.

Python Version:
Tested in Python 3.12.4 (tags/v3.12.4:8e8a4ba, Jun 6 2024, 19:30:16) [MSC v.1940 64 bit (AMD64)]

Why This Matters:

1. Metaclass usage is already an advanced topic.
2. Improving the DX (developer experience) here will help advanced learners and reduce friction.
3. Similar improvements have already been adopted in recent Python versions for other common errors.

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

先重現 issue 中描述的兩個 metaclass new 情況，並追蹤引發目前 TypeError 的執行階段進入點。更新缺少回傳值或回傳值不是類別時的診斷資訊，接著為兩個範例加入涵蓋範圍，並確認訊息清楚說明所需的回傳值。