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

まず doctest モジュールと、既存の ELLIPSIS の処理を読みます。リテラルな省略記号マーカーをどのように指定すべきか、また別の doctest フラグが必要かどうかを判断し、その後、提案するマーカーがリテラルな省略記号に一致し、通常の省略記号のマッチングも引き続き機能することを示すリグレッションテストのカバレッジを追加します。