`KeyError` hides the exception raised by str(key)

Bug report

Bug description:

>>>a = object.__new__(type(exit))
>>>b = {}
>>>b[a]
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    b[a]
    ~^^^
KeyError: <exception str() failed>

The exception AttributeError was hidden:

Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    b[a]
    ~^^^
AttributeError: 'Quitter' object has no attribute 'name'

The exception in str() shouldn't be ignore. May because of the lack of the attribute or the wrong type that __str__ return.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

Linked PRs

• gh-139961

先在 CPython 3.13 上重現報告中的 b[a] 範例，並檢查 KeyError 訊息如何呼叫 str(key)。查看連結的 PR gh-139961 以了解目前的方向；完成的標準是，格式化 key 時引發的 exception 會被保留，而不是被 KeyError 的顯示隱藏。