rtd uses sphinx 1.8.5 even if it previously installed 2.2.0

[ssbarnea]

Details

• Read the Docs project URL: https://github.com/pycontribs/jira/blob/master/.readthedocs.yml
• Build URL (if applicable): https://readthedocs.org/projects/jira/builds/9803343/
• Read the Docs username (if applicable): ssbarnea

Expected Result

Successful build of docs using the requires sphinx 2.2.0.

Actual Result

Based on the logs sphinx 2.2.0 is installed (docs extra) but after that I see:

python /home/docs/checkouts/readthedocs.org/user_builds/jira/envs/latest/bin/sphinx-build -T -E -b readthedocs -d _build/doctrees-readthedocs -D language=en . _build/html

 Running Sphinx v1.8.5
loading translations [en]... done

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/jira/envs/latest/lib/python3.7/site-packages/sphinx/cmd/build.py", line 303, in build_main
    args.tags, args.verbosity, args.jobs, args.keep_going)
  File "/home/docs/checkouts/readthedocs.org/user_builds/jira/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 215, in __init__
    'be built with this version.') % self.config.needs_sphinx)
sphinx.errors.VersionRequirementError: This project needs at least Sphinx v2.2.0 and therefore cannot be built with this version.

Sphinx version error:
This project needs at least Sphinx v2.2.0 and therefore cannot be built with this version.

Apparently the sphinx-build is called without activating the virtual environment, so failing

[humitos]

You probably need to wipe the environment. Can you try that first? Thanks.

[stsewd]

This looks like a cache problem, please try wiping the env before triggering a new build https://docs.readthedocs.io/en/stable/guides/wipe-environment.html

[matfax]

This is probably a duplicate of #5810