doctest: Allow checking for Traceback context errors

Feature or enhancement

Proposal:

If the error thrown in a doctest command is a ValueError, and that error object has a __context__ of RuntimeError, it would be nice to be able to do this:

>>> function_that_raises()
Traceback (most recent call last):
  ...
RuntimeError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  ...
ValueError

And a similar thing for __cause__ but with the different line of text in the middle.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

I did make a discourse post, but it only got a like and no comments :)

从 doctest 的 traceback 检查实现及其现有测试开始；该 issue 未提供文件路径。重现所示的 context 情况，并将其与提议的 cause 变体进行比较，然后为所请求的中间 traceback 文本添加覆盖测试，并验证 doctest 输出。