Build error with build id #21850791

[ots22]

Details:

• Project URL: https://readthedocs.org/projects/scivision/
• Build URL(if applicable): https://readthedocs.org/projects/scivision/builds/21850791/
• Read the Docs username(if applicable): ots22

Expected Result

Build to complete successfully

Actual Result

The last step of the build that completes is

python -m pip install --upgrade --no-cache-dir pillow mock==1.0.1 alabaster>=0.7,<0.8,!=0.7.5 commonmark==0.9.1 recommonmark==0.5.0 sphinx sphinx-rtd-theme readthedocs-sphinx-ext<2.3

The next step doesn't give any output and the build errors at that point with a generic message ('There was a problem with Read the Docs while building your documentation.')

[ots22]

The error seemed to be caused by having

extra_requirements:
  - [doc]

in .readthedocs.yml, instead of

extra_requirements:
  - doc

[humitos]

@ots22 there is an example in the documentation about this config key: https://docs.readthedocs.io/en/stable/config-file/v2.html#packages

However, the error shown to the user should be pretty clear. I'm adding this to #10502