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

先閱讀 doctest 模組及其現有的 ELLIPSIS 處理。確定應如何指定字面省略號標記，以及是否需要單獨的 doctest 旗標，然後新增回歸測試涵蓋範圍，證明所提出的標記可以比對字面省略號，同時一般的省略號比對仍然有效。