Support `<LITERAL ELLIPSIS>` in `doctest` module

Feature or enhancement

Proposal:

Currently, with # doctest: +ELLIPSIS, a ... can match anything. Sometimes, we want to test that a test matches a literal ellipsis.

I propose something similar to <LITERAL ELLIPSIS>. Possibly guarded behind another doctest flag.

'''
>>> '123...456' # doctest: +ELLIPSIS
'123<LITERAL ELLIPSIS>456'
'''

Motivation: SageMath doctest. See https://github.com/sagemath/sage/pull/41276, https://github.com/sagemath/sage/pull/41182.

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

先阅读 doctest 模块及其现有的 ELLIPSIS 处理。确定应如何指定字面省略号标记，以及是否需要单独的 doctest 标志，然后添加回归覆盖，证明所提出的标记可以匹配字面省略号，同时普通的省略号匹配仍然有效。