readthedocs started to throw an error Sphinx version error during build, although there were no changes to the readthedocs configuration and dependencies are all pinned.
Here's the repository and a specific [PR] https://github.com/dask/dask-image/pull/344 for which the doc build fails. Here's the error message in the readthedocs build:
(shortened)
Running Sphinx v4.5.0
loading translations [en]... done
...
Sphinx version error:
The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.
HOWEVER, sphinx is already pinned to >5 in the repository, but version 4.5 seems to be installed during doc build.
The error looks very similar to this question, however in my case there's no such misconfiguration and the fully pinned doc environment defined in continuous_integration/environment-doc.yml hasn't changed since the fails started.
What's interesting is that cat continuous_integration/environment-doc.yml on the readthedocs shows something different than continuous_integration/environment-doc.yml in the repository. Namely, there's a conda dependency sphinx added at the end, as if readthedocs added lines in the background.
Any help is greatly appreciated!