I agree that it should be easier to keep them all in sync.  But my point is that the on-going official doc builds (some multi[ple times per day) for all of the active versions (2.7 and 3.6 through 3.9) are using 2.3.1 so we should be doing CI against that version as well.  The docs builds are configured in https://github.com/python/docsbuild-scripts.  So whenever requirements.in is update there, the CI versions should be updated as well. Even better would be someway to auto sync them.

Julien, any ideas on this?