3.9 & 3.10: Read The Docs complains about missing config key

[encukou]

On a backport PR to 3.10 (e.g. #129440), and presumably to 3.9 too, Read the Docs will complain about a missing config key (before it gets to the exit 183 telling it to not build anything for that branch). This shows up as a failed check in GHA.

Linked PRs

• [3.10] gh-129509: Add required key to the dummy Read the Docs config file #129510
• [3.9] gh-129509: Add required key to the dummy Read the Docs config file (GH-129510) #130249