label of deprecated-removed directive is not translatable

[cocoatomo]

A label of deprecated-removed directive is translatable when I submitted this PR, but become not translatable now.
Also, impl-detail directive is.

The related .po file: https://github.com/python/python-docs-ja/blob/3.8/sphinx.po#L44

[tk0miya]

So far, custom extensions for python-doc have accessed internal data structure of sphinx.locale module directly. But, it does not work at present because the structure has changed since Sphinx-1.8 release.

I made a patch for using public translation APIs of Sphinx instead to get translated messages. Could you patch this to the extensions please?
https://gist.github.com/e2de1962f073d389251ba13e285b4336