Adds readthedocs yml file in order to specify python version for builds

This resolves the build failure that happened after merging the changes from !219 (merged)

JIRA tickets: SE-3944

Sandbox URL: https://handbook.opencraft.com/en/nizar-se-3944-fix-builds/

Merge deadline: ASAP

Testing instructions:

1. Check the build#12853181 and verify that it succeeded.
2. Check the sandbox url and make sure the cells page renders properly.

Reviewers

• @FoxyFoxie