.. only:: not working in gettext builder

[nijel]

Is your feature request related to a problem? Please describe.
#2874 described reasons why .. only:: should be ignored in gettext builder and #2878 implemented that. I see two issues with that:

• This behavior is not documented (besides note in changelog entry from 2016).
• This makes it impossible to exclude certain content from being localized.

Describe the solution you'd like

Let's take this rst:

.. only:: not gettext

   Content not to be localized.

.. only:: html

   Content to be localized.

I'd like the first block not to be extracted and the second one to be.

Describe alternatives you've considered

Any other syntax for excluding content from the generated POT file would do the job as well. Looking at the documentation the obvious way would be using .. only:: and its documentation does not contain any mention that it does not work with the gettext builder.

[nijel]

Workaround I'm currently using:

import sphinx.builders.gettext
from sphinx.util.tags import Tags


class WeblateTags(Tags):
    def eval_condition(self, condition):
        # Exclude blocks marked as not gettext
        return condition != "not gettext"


def setup(app) -> None:
    # Monkey path gettext build tags handling, this is workaround until
    # https://github.com/sphinx-doc/sphinx/issues/13307 is addressed.
    sphinx.builders.gettext.I18nTags = WeblateTags

It is definitely not something ready for the inclusion as the text comparing is fragile, but doing something similar and more robust in _eval_node should be doable. I can contribute a PR if this is something acceptable.