Gettext builder not extracting content of doctest blocks

[m-aciek]

Describe the bug

Hi, running a gettext builder on a documents with doctest-block in gettext_additional_targets doesn't add the content of doctest blocks in generated POTs.

How to Reproduce

1. Add doctest-block to gettext_additional_targets in conf.py
   (gh-123407: Allow translation of doctest blocks content python/cpython#123852).

2. Run % make gettext.

3. -> lack of expected doctest blocks in POTs, example one: https://github.com/python/cpython/blob/be90648fb2de58b148dcc7553a08ca646911baf2/Doc/tutorial/introduction.rst?plain=1#L150-L155

Environment Information

Sphinx version: v7.3.7

Additional context

From a quick look I'm not able to figure out the cause in the Sphinx codebase.